How to charge and monitor 12V sealed/gel lead acid batteries?

February 8, 2017 6:49 PM

I used to have a cheap battery charger which would supply a selectable 2, 4, or 6 Amps into a 12 V lead-acid battery and then automatically shut off once it decided the battery was charged. Somehow, I misplaced this charger and am looking for a more feature-ful replacement.

Since it seems I need to replace this thing, I would like to replace it with something that is a bit "smart"er than the cheapo unit I had before. If possible, I would like something which can do at least the first, and preferably both of the following:

1) When asked, fully charge a battery, then discharge the battery at a controlled rate until it reaches a certain discharge voltage, so as to report the current working capacity of the battery.

2) Be able to monitor the status of a battery that is being used to power a load, and pass power through to the load as needed, and also maintain the charge status of the battery as needed, for instance if the electric power supplying the charger fails for a period of time.

Any suggestions which meet either or both of these criteria will be appreciated. Thanks!

posted by Juffo-Wup to Technology (6 answers total)

Gel cells are harder to "gas gauge" than lithium ion cells. You might have noticed how accurate state of charge monitoring and lithium batteries both showed up in consumer devices around the same time. This was not a coincidence. Lithium cells are much easier to track.

The biggest problem you will encounter is that the capacity of a gel cell is strongly dependent on the rate of discharge. For example, if you discharge a gel cell slowly over 20 hours, you'll get double the Ah out of it than if you try to discharge it in 30 minutes. See: Peukert's law.

This isn't to say that you won't be able to find a charger that does what you want. Just be aware that it will never be as accurate as your cell phone battery gauge. But if your load is pretty constant, you should be able to figure out a rough runtime.

tldr: gel cells are sort of crap
posted by ryanrs at 10:43 PM on February 8, 2017

Now for something possibly more helpful. Here are three groups that still use lead acid batteries. Maybe this will help direct your search.

Industrial vehicles. Electric forklifts, stuff like that. Also golf carts. Companies that make charging systems for these things might have stuff for smaller gel cells too.

DIY electric cars. Yes, this is a thing, and has been for a long, long time. It's a pretty active scene and they have been working with lead-acid cells forever. There's a mailing list. There are also a lot of small companies making battery chargers / conditioners / monitors for lead-acid batteries in home-built electric cars. Check out Manzanita Micro. They might have something useful or at least point you in the right direction.

Amateur radio groups. I don't have any links handy, but they have a thing called Field Day that's basically a holiday celebrating gel cells.
posted by ryanrs at 10:59 PM on February 8, 2017

You want to 'condition' your battery.
This may be what your looking for...
BatteryMINDer 12V 2/4/8 Amp Charger / Maintainer / Desulfator

(But I simply use a Battery Tender Jr (022-0192), cheap, no frills, easy-peazy to keep the Ducati's battery charged & maintained in the off season).
posted by artdrectr at 10:59 PM on February 8, 2017

Can you clarify what the application is here? I ask because...

and also maintain the charge status of the battery as needed, for instance if the electric power supplying the charger fails for a period of time.

...seems contradictory - how can a battery charger "maintain the charge status... if the electrical power...fails".

Do you want a battery charge status monitor that is powered by the 12V battery itself? Confused.
posted by soylent00FF00 at 6:19 PM on February 9, 2017

The key word you are looking for is in number 1 is "maintainer". Number 2 is just using the battery charger as a power supply, which should work, but I can see that it might throw off some of the diagnostic features of fancier chargers.

To fulfill all of your requirements, you'll want a combination charger/maintainer. Schumacher is a reasonably decent brand that is widely available. This one might do what you are looking for, and is currently cheaper than the similar model with lower output on Amazon.

You don't have to use the clamp style terminals either, you can get a set of ring terminals that permanently attach to your battery and can be unplugged from the charger if needed.
posted by ArgentCorvid at 6:41 AM on February 10, 2017

adding to ryanrs tip: power wheelchairs use lead-acid batteries.
posted by Jesse the K at 11:04 AM on February 10, 2017