Help us safely charge our 12v marine/rv battery system
May 31, 2022 2:59 PM Subscribe
After a long overdue replacement of our boat's batteries, we have concerns that our charging system might be overcharging the new ones. We have questions about what the appropriate behavior should be. Lots of details below the fold.
We have two battery banks on our boat as part of the 12V DC system. The house bank is two 6V 400Ah batteries wired in series. The engine bank is two 12V 100Ah wired in parallel. The system is charged by 1) a shore power converter, 2) a solar charge controller, and 3) the alternator hooked to a regulator.
Over the past year, we found our flooded lead acid batteries got hungrier and hungrier for water, and, on greater inspection, that they were over 10 years old. As far as we can tell they were in a cycle of never being able to be fully charged, so the chargers kept overcharging them and boiling the water off. (It's a miracle any electronics on the boat worked, especially cranking the engine.) We made the decision to replace everything with AGMs. We're still sourcing the house batteries (thanks supply chain issues), but got the two engine batteries, which is enough for both our engine and house needs for the time being. We adjusted the settings on all the chargers from flooded to AGM.
Here's the issue: The batteries came fully charged, which we verified with a battery tester: voltage was 12.8, state of charge 100%, and state of health 100%. But when connected to either the shore power converter of the solar charge controller (we tested them independently), neither registered as "charged" or "full," and they continued delivering 13.7V float voltage even after a couple hours of charging. After disconnecting the chargers, the battery voltage read 13.1V which by all definitions seems to be overcharged. I had to leave the batteries in that state of charge overnight, and when I returned this morning, the batteries read 12.83V.
Key questions: Why are our chargers continuing to charge our fully-charged batteries? Are we correct to assume they should shut off and read charged when the battery is 12.8V? If not, what should we expect them to charge the batteries to? Will it be safe? If this is unexpected behavior, what should we try next—debugging, replacing chargers, etc?
Also, how badly could I have hurt the batteries by leaving them at 13.1V? Why did they drop overnight to 12.8V?
Bonus question: according to the alternator regulator manual, it bulk charges the batteries for 23 minutes after starting the engine no matter what state of charge the batteries are in. Is this standard? Will this hurt the batteries, or is this time/voltage determined to be what's needed after cranking the engine? One would think with 200Ah, cranking the engine would not take the batteries so far down as to require a bulk charge.
tl;dr solar and shore power never read fully charged and continue to float when brand new batteries are at or above 12.8V what do
We have two battery banks on our boat as part of the 12V DC system. The house bank is two 6V 400Ah batteries wired in series. The engine bank is two 12V 100Ah wired in parallel. The system is charged by 1) a shore power converter, 2) a solar charge controller, and 3) the alternator hooked to a regulator.
Over the past year, we found our flooded lead acid batteries got hungrier and hungrier for water, and, on greater inspection, that they were over 10 years old. As far as we can tell they were in a cycle of never being able to be fully charged, so the chargers kept overcharging them and boiling the water off. (It's a miracle any electronics on the boat worked, especially cranking the engine.) We made the decision to replace everything with AGMs. We're still sourcing the house batteries (thanks supply chain issues), but got the two engine batteries, which is enough for both our engine and house needs for the time being. We adjusted the settings on all the chargers from flooded to AGM.
Here's the issue: The batteries came fully charged, which we verified with a battery tester: voltage was 12.8, state of charge 100%, and state of health 100%. But when connected to either the shore power converter of the solar charge controller (we tested them independently), neither registered as "charged" or "full," and they continued delivering 13.7V float voltage even after a couple hours of charging. After disconnecting the chargers, the battery voltage read 13.1V which by all definitions seems to be overcharged. I had to leave the batteries in that state of charge overnight, and when I returned this morning, the batteries read 12.83V.
Key questions: Why are our chargers continuing to charge our fully-charged batteries? Are we correct to assume they should shut off and read charged when the battery is 12.8V? If not, what should we expect them to charge the batteries to? Will it be safe? If this is unexpected behavior, what should we try next—debugging, replacing chargers, etc?
Also, how badly could I have hurt the batteries by leaving them at 13.1V? Why did they drop overnight to 12.8V?
Bonus question: according to the alternator regulator manual, it bulk charges the batteries for 23 minutes after starting the engine no matter what state of charge the batteries are in. Is this standard? Will this hurt the batteries, or is this time/voltage determined to be what's needed after cranking the engine? One would think with 200Ah, cranking the engine would not take the batteries so far down as to require a bulk charge.
tl;dr solar and shore power never read fully charged and continue to float when brand new batteries are at or above 12.8V what do
Yes exactly this. Don't make any assumptions about what batteries "should" do or give. Read the actual manufacturers notes on that specific battery regarding charge profiles, and set your charging system to match what the batteries want. A "fully charged" battery might read 12.8v at rest # or 12.2v if you're pulling 100A from it, or well over 13v if the charger is connected. It's almost impossible to "overcharge" agm batteries with a reasonable quality charger, and certainly not by just leaving it all connected, which is what the batteries really want you to do.
posted by tillsbury at 7:18 PM on May 31, 2022
posted by tillsbury at 7:18 PM on May 31, 2022
This seems like expected behaviour to me. The unexpected behaviour for batteries in good operating order would be the behaviour you experienced with the old batteries.
Modern battery chargers often stay in absorption mode until around the 14.4v stated and pointed out by iamabot and, where they are connected long-term, usually sit around 13.5v or so in float mode. This will vary if, for example, you're drawing a load from the battery and you'll see the voltage drop in that case. Both the solar and the shore power chargers will remain in float mode indefinitely unless something changes the charge state (eg you use power).
The regulator actually has fixed periods of 36 minutes bulk, followed by 120 minutes absorption (according to p3 of the manual you linked). You can change the time periods as explained later in the manual, although it's a bit of a weird process. Don't forget the engine batteries are being constantly drained to a greater or lesser extent, depending on what type of engines you have and what else may run off those batteries (usually nothing, but don't assume). These charging times do seem overly long, but charging takes a lot longer than discharging in an environment where the discharge is very high, albeit for brief periods. I would be inclined to investigate reducing the charge periods, but it's not a pressing need by any means.
posted by dg at 8:41 PM on May 31, 2022
Modern battery chargers often stay in absorption mode until around the 14.4v stated and pointed out by iamabot and, where they are connected long-term, usually sit around 13.5v or so in float mode. This will vary if, for example, you're drawing a load from the battery and you'll see the voltage drop in that case. Both the solar and the shore power chargers will remain in float mode indefinitely unless something changes the charge state (eg you use power).
The regulator actually has fixed periods of 36 minutes bulk, followed by 120 minutes absorption (according to p3 of the manual you linked). You can change the time periods as explained later in the manual, although it's a bit of a weird process. Don't forget the engine batteries are being constantly drained to a greater or lesser extent, depending on what type of engines you have and what else may run off those batteries (usually nothing, but don't assume). These charging times do seem overly long, but charging takes a lot longer than discharging in an environment where the discharge is very high, albeit for brief periods. I would be inclined to investigate reducing the charge periods, but it's not a pressing need by any means.
posted by dg at 8:41 PM on May 31, 2022
Best answer: Lead-acid batteries cope well with a wide range of charging currents.
Modern chargers usually do the job in several phases. First phase runs in constant current mode, where the charger supplies the maximum amperage it's rated for into the battery until the terminal voltage rises to about 2.4V per cell, so 14.4V for a nominally 12V six-cell battery.
Next there's constant voltage mode, where 2.4V per cell gets maintained until the charging current drops to a fraction of its initial value, which it will do even though 2.4V per cell is still well over the open-circuit voltage you'd measure across a fully charged cell that's been sitting disconnected for a few hours.
Next comes float charge mode, another constant voltage phase run at about 2.3V per cell. This makes the charging current drop quite substantially. A fully charged lead-acid battery connected to a constant-voltage charger applying 2.3V float voltage per cell will happily sip the resulting small current forever, and will also recover reasonably quickly from brief discharge demands. It won't draw anywhere near enough charging current to make it gas. All it will do with excess charging current is turn it into heat: not much, but often enough to stop a battery from freezing in a cold environment.
Some chargers will monitor the current being drawn in float mode, and if they see it remain low and constant for a few hours, will then drop the per-cell voltage down again, to 2.25V or even 2.2V per cell. This is trickle charge mode, and again any lead-acid battery will cope with having this applied indefinitely. The only difference is that the resulting charge current will typically be about a tenth of the already low rate it was during float charge, reducing the amount of energy devoted to doing nothing more useful than warming the battery. The purpose of trickle charge mode is to supply just the slightest bit more charge current than the battery needs to compensate for its own self-discharge current.
If you fully charge a lead-acid battery and then disconnect it from the charger and leave it sit for a few hours, it will self-discharge moderately quickly until the per-cell voltage drops to about 2.1V at which point the self-discharge current will be about as low as it's going to stay. So you'll typically see a recently charged, nominally 12V battery read about 12.6V into an open circuit for maybe a week if it's still in good nick.
As long as your batteries are within a few percent of any of these voltages then they're behaving as expected, and you can just not worry too much about what the charge controllers say their actual state of charge is as long as it's well over 50% almost all of the time.
State-of-charge evaluation can be quite wonky, especially once the battery starts getting full (because charging voltage and currents get pretty stable then) and double especially if you've got multiple regulated charge controllers all feeding into the battery at once via isolating diodes to stop their regulators from fighting each other. It's quite common for "smart" charge controllers to evaluate a battery's state of charge by briefly ceasing to charge it, dropping a small load across it instead, and measuring the voltage that the now-discharging battery develops across that load. But if you've got an isolating diode between the charge controller and the battery then the battery won't be able to feed any current back into the test load and the controller will get a bogus voltage measurement that it will probably just ignore.
according to the alternator regulator manual, it bulk charges the batteries for 23 minutes after starting the engine no matter what state of charge the batteries are in. Is this standard? Will this hurt the batteries, or is this time/voltage determined to be what's needed after cranking the engine? One would think with 200Ah, cranking the engine would not take the batteries so far down as to require a bulk charge.
Starter motors are hungry brutes, and cranking an engine will typically involve a battery discharge current of around 250A. Bulk charging is usually done at around C/10 for the chonkiest battery supported by the regulator, so maybe 20A for a regulator designed to work with 200Ah batteries. Bulk charging for long enough to replace all the charge consumed by cranking the engine should therefore take (250A discharge rate / (20A charge rate * 80% round-trip efficiency)) = about 15 times as long as the cranking event itself lasted for, and this will be the case regardless of the battery's initial state of charge.
23 minutes, then, is a compromise. It's going to supply enough bulk charge to pay back a battery that's been cranking away at an engine for a minute and a half, plus a bit extra to account for most starter batteries not being fully charged on most starts due to typically having sat for some time doing nothing but self-discharge and maybe run a clock or a radio or whatnot.
This will indeed typically be more than is strictly needed - often, in fact, quite a lot more - but not enough more to get anywhere near cooking the battery. Just a good amount that will cover most people's use cases without anybody needing to think too hard about it.
posted by flabdablet at 10:09 AM on June 1, 2022 [1 favorite]
Modern chargers usually do the job in several phases. First phase runs in constant current mode, where the charger supplies the maximum amperage it's rated for into the battery until the terminal voltage rises to about 2.4V per cell, so 14.4V for a nominally 12V six-cell battery.
Next there's constant voltage mode, where 2.4V per cell gets maintained until the charging current drops to a fraction of its initial value, which it will do even though 2.4V per cell is still well over the open-circuit voltage you'd measure across a fully charged cell that's been sitting disconnected for a few hours.
Next comes float charge mode, another constant voltage phase run at about 2.3V per cell. This makes the charging current drop quite substantially. A fully charged lead-acid battery connected to a constant-voltage charger applying 2.3V float voltage per cell will happily sip the resulting small current forever, and will also recover reasonably quickly from brief discharge demands. It won't draw anywhere near enough charging current to make it gas. All it will do with excess charging current is turn it into heat: not much, but often enough to stop a battery from freezing in a cold environment.
Some chargers will monitor the current being drawn in float mode, and if they see it remain low and constant for a few hours, will then drop the per-cell voltage down again, to 2.25V or even 2.2V per cell. This is trickle charge mode, and again any lead-acid battery will cope with having this applied indefinitely. The only difference is that the resulting charge current will typically be about a tenth of the already low rate it was during float charge, reducing the amount of energy devoted to doing nothing more useful than warming the battery. The purpose of trickle charge mode is to supply just the slightest bit more charge current than the battery needs to compensate for its own self-discharge current.
If you fully charge a lead-acid battery and then disconnect it from the charger and leave it sit for a few hours, it will self-discharge moderately quickly until the per-cell voltage drops to about 2.1V at which point the self-discharge current will be about as low as it's going to stay. So you'll typically see a recently charged, nominally 12V battery read about 12.6V into an open circuit for maybe a week if it's still in good nick.
As long as your batteries are within a few percent of any of these voltages then they're behaving as expected, and you can just not worry too much about what the charge controllers say their actual state of charge is as long as it's well over 50% almost all of the time.
State-of-charge evaluation can be quite wonky, especially once the battery starts getting full (because charging voltage and currents get pretty stable then) and double especially if you've got multiple regulated charge controllers all feeding into the battery at once via isolating diodes to stop their regulators from fighting each other. It's quite common for "smart" charge controllers to evaluate a battery's state of charge by briefly ceasing to charge it, dropping a small load across it instead, and measuring the voltage that the now-discharging battery develops across that load. But if you've got an isolating diode between the charge controller and the battery then the battery won't be able to feed any current back into the test load and the controller will get a bogus voltage measurement that it will probably just ignore.
according to the alternator regulator manual, it bulk charges the batteries for 23 minutes after starting the engine no matter what state of charge the batteries are in. Is this standard? Will this hurt the batteries, or is this time/voltage determined to be what's needed after cranking the engine? One would think with 200Ah, cranking the engine would not take the batteries so far down as to require a bulk charge.
Starter motors are hungry brutes, and cranking an engine will typically involve a battery discharge current of around 250A. Bulk charging is usually done at around C/10 for the chonkiest battery supported by the regulator, so maybe 20A for a regulator designed to work with 200Ah batteries. Bulk charging for long enough to replace all the charge consumed by cranking the engine should therefore take (250A discharge rate / (20A charge rate * 80% round-trip efficiency)) = about 15 times as long as the cranking event itself lasted for, and this will be the case regardless of the battery's initial state of charge.
23 minutes, then, is a compromise. It's going to supply enough bulk charge to pay back a battery that's been cranking away at an engine for a minute and a half, plus a bit extra to account for most starter batteries not being fully charged on most starts due to typically having sat for some time doing nothing but self-discharge and maybe run a clock or a radio or whatnot.
This will indeed typically be more than is strictly needed - often, in fact, quite a lot more - but not enough more to get anywhere near cooking the battery. Just a good amount that will cover most people's use cases without anybody needing to think too hard about it.
posted by flabdablet at 10:09 AM on June 1, 2022 [1 favorite]
We made the decision to replace everything with AGMs
You may already realize this, but AGMs can dry out as well. Except unlike flooded lead-acid batteries, there's really no way to replenish the fluid when it does. The batteries just die. In the short term they are "maintenance free", but you will probably have to replace them more often than flooded types.
I actually went the opposite direction, and have gotten rid of most of my AGMs in my larger UPSs and such, because I'd rather have flooded batteries that I can check the levels on periodically and top off.
Others have given a lot of good information about bulk/absorption/float charging, so the only other thing I'll add is that adding a temperature sensor to your setup, so you can make sure the batteries aren't overheating, might be desirable. I've yet to see a controller that doesn't cost a spectacular amount (there are some very high-end marine integrated charge/maintain/inverter systems that do IIRC) that actually takes temperature into account, though. So you might need to set up some system to alert you if the temperature exceeds ambient by more than a certain amount, so you can shut the system down or engage a ventilation fan or take some other action manually.
posted by Kadin2048 at 1:36 PM on June 1, 2022 [1 favorite]
You may already realize this, but AGMs can dry out as well. Except unlike flooded lead-acid batteries, there's really no way to replenish the fluid when it does. The batteries just die. In the short term they are "maintenance free", but you will probably have to replace them more often than flooded types.
I actually went the opposite direction, and have gotten rid of most of my AGMs in my larger UPSs and such, because I'd rather have flooded batteries that I can check the levels on periodically and top off.
Others have given a lot of good information about bulk/absorption/float charging, so the only other thing I'll add is that adding a temperature sensor to your setup, so you can make sure the batteries aren't overheating, might be desirable. I've yet to see a controller that doesn't cost a spectacular amount (there are some very high-end marine integrated charge/maintain/inverter systems that do IIRC) that actually takes temperature into account, though. So you might need to set up some system to alert you if the temperature exceeds ambient by more than a certain amount, so you can shut the system down or engage a ventilation fan or take some other action manually.
posted by Kadin2048 at 1:36 PM on June 1, 2022 [1 favorite]
This thread is closed to new comments.
>Why are our chargers continuing to charge our fully-charged batteries?
Because that's what they are supposed to do, they maintain the batteries charge state. It looks like your charge controllers have a variety of profiles you should check - some of which are influenced by temperature:
Bulk Charge In the first stage, known as the bulk charge, Truecharge+ delivers its fullrated output current.This constant current is delivered to the batteries until
the battery voltage approaches its absorption voltage—typically around
14.4 volts for 12 volt batteries. The bulk charge stage restores about 75%
of the battery’s charge.
Absorption
Charge
During the absorption charge, the charging voltage is held constant near
the gassing voltage, and the battery gradually reduces the charging current
it demands as it attains full charge. Once the current drops below 4 Adc
for three minutes, the charger either exits to Float for gel and AGM
batteries or charges flooded batteries for one more hour.
This extended one-hour charge ensures that the batteries are fully topped
up and is recommended by the flooded battery manufacturers.
Float Charge The float charge is a maintenance mode in which the output voltage of the
charger is reduced to a lower level, typically about 13.5 volts to maintain
the battery’s charge without losing electrolyte through gassing. Unlike
many standalone chargers, Truecharge+ can be left connected to your
battery indefinitely without risk of overcharging.
In the float mode, the charger will initiate a new charge cycle under any of
these conditions:
• AC power is disconnected and reconnected
• any battery drops below 12.5 VDC for 15 minutes
• 21 days have passed from the time the charger completed Absorption.
The chart in Figure B-1 shows the three-stage charging profile
>Are we correct to assume they should shut off and read charged when the battery is 12.8V?
No, you are incorrect, your charge controllers will continue to float voltage into your battery as long as they are hooked up to the battery and an external power source and other conditions for that profile are met.
> If not, what should we expect them to charge the batteries to?
It's in the manuals for the products., depends on a few things.
>Will it be safe?
Probably
> If this is unexpected behavior, what should we try next—debugging, replacing chargers, etc?
It seems like expected behavior to me.
Your experience with your old batteries was a factor of a few things - they were old, and thus probably weren't capable of accepting the proper charge leading to the cycle of evaporative loss - they weren't necessarily being excessively charged, they were probably past their service life.
You also moved from wet lead acid batteries, which your charge controllers will treat differently. I think it's pretty unlikely that your batteries are being over charged, with AGM's it's more typical that they are _under charged_ in their bulk phase of the charging cycle.
posted by iamabot at 3:41 PM on May 31, 2022 [3 favorites]