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A battery of questions!
October 26, 2011 3:42 PM   Subscribe

Does an expensive battery charger do a better job of charging and maintaining NiMH batteries than a generic, cheapo charger?

A couple of years ago, I purchased a La Crosse Technology BC-700 Alpha Power Battery Charger (linked here : http://www.amazon.com/gp/product/B000RSOV50 )

The reviews were very positive, and I purchased one, based on these reviews, and the fact that I seemed to have accumulated old NiMH batteries that eventually seemed to become less and less useful. Upon putting some very old NiMH batteries in the LaCrosse charger, it ran through a lengthy process of discharging and recharging, and by god, it seemed to bring them back to life! According to the product FAQ,

"The REFRESH mode will discharge and recharge the battery up to 20 times to reach its fullest capacity. The REFRESH or conditioning mode works best on batteries that have been used under load for awhile first. It is also a great way to renew old rechargeable
batteries"

I have been happy with the charger, but when I mentioned it to a friend (who, is much MUCH more knowledgeable about such things), he commented that,

"I had recently been doing a lot of research on charging NiMH for a friend who asked me to design a special charger for him (to keep batteries topped-up in long-term storage) - general consensus is that discharging NiMHs in a charge cycle is a BAD thing. It unnecessarily reduces battery life with no known advantage"

So, is my charger using some sort of flim-flam and harming my batteries? I don't think I've lost any batteries in the 2 years that I've owned the device and have been very happy with it, so I'm a little confused.

Am I simply the happy owner of a decent battery charger, or am I a victim of some sort of electronic placebo effect?
posted by newfers to Technology (7 answers total) 6 users marked this as a favorite
 
The point of a full discharge before recharge is to avoid "memory effect". LiIon batteries don't suffer much from memory effect, so there's no benefit in doing that for LiIon.

But NiMH batteries are very strongly susceptable to memory effect, and contra what your friend says, an occasional full discharge is good for them.

I suspect your friend got confused by the fact that LiIon batteries don't need this.
posted by Chocolate Pickle at 3:49 PM on October 26, 2011


My friend Darrell said :
"I've read many app notes by some major electronic component manufactures who offer battery management products, and all I found so far suggest to NOT discharge NiMHs before charging - only NiCds."

He also posted the following quote from Microchip's App Note (not sure what that is!) :

"Voltage depression or “memory” has always been a problem in applications using nickel-cadmium cells. When nickel-cadmium cells are routinely partially discharged, a depression of about 150 mV has been reported to appear in the discharge voltage profile. The severity of this problem is open to interpretation, but it is agreed that the cause of the problem is in the structure of the cadmium electrode. Since the nickelmetal hydride cells use hydrogen-absorbing alloy instead of cadmium, voltage depression is no longer a concern".
posted by newfers at 3:57 PM on October 26, 2011


You wouldn't want to fully discharge NiMH every time you charge. But it's worth doing once every few months.

NiCads are legendary for this, but other battery technologies suffer from it to lesser extents.

Here's a link:
Technically, NiMH batteries do not have a "memory effect", but strictly speaking neither do NiCds. However NiMH batteries can experience voltage depletion, also called voltage depression, similar to that of NiCd batteries, but the effect is normally less noticeable. To completely eliminate the possibility of NiMH batteries suffering any voltage depletion effect manufacturers recommend an occasional, complete discharge of NiMH batteries followed by a full recharge. NiMH batteries can also be damaged by overcharge and improper storage (see the NiCd section immediately above this one). Most users of NiMH batteries don't have to be concerned with this voltage depletion effect. But if you use a device say a flashlight, radio, or digital camera for only a short time every day and then charge the batteries every night, you will need to let the NiMH (or NiCd) batteries run down occasionally.
posted by Chocolate Pickle at 4:04 PM on October 26, 2011


Can't answer your exact question but I've read that the big deep cycle batteries for marine or off-grid significantly benefit from a quality 'smart' charter due to voltage levels and sensing when to stop the charge. I hope we have a battery guy that chimes in.
posted by sammyo at 4:45 PM on October 26, 2011


It definitely makes a difference. I quote from from the following site (yes, they sell fancy chargers, but the information to the best of my knowledge is true). FYI this completely accords with my own personal experience.

"Getting the highest performance from rechargeable batteries depends largely on the quality of the battery charger. Instead of using a timer to provide a fixed amount of charge given to a battery, all [brand name] Battery Chargers are smart chargers.

"Some AA battery chargers are just timer based chargers. A timer charger cannot tell if batteries are empty or full, or in between. A timer charger assumes the batteries are empty, and the battery charger will charge the batteries for a set amount of time. This will give the batteries more charge than they need if they are partly charged. This overcharging will heat the batteries and will reduce the number of recharge cycles you will get from the batteries during their lifetime.

"This is particularly true of the new low self discharge batteries, as they are often not empty when recharged. Similarly, timer chargers do not provide a full charge to today's higher capacity AA batteries, as the fixed amount of charge was designed for older, lower capacity AA batteries."
posted by smoke at 4:51 PM on October 26, 2011


Even if you have no understanding of electricity or chemistry, you can learn quite a lot about the charge / discharge characteristics & care and feeding of batteries of various types by reading the various manufacturer's appnotes. Microchip's appnotes tend to be particularly good in this respect, with fairly simple-english intro sections and a bunch of pretty & explanatory graphs ;-)

For example, here's one for a Microchip NiMH charge controller chip, and one for a multi-chemistry reference design. Couldn't quickly find one for a NiCd charger chip - nobody makes just NiCd chargers anymore - but their charge characteristics (e.g. end-point voltage drop, temperature, etc) are like a more pronounced version of a NiMH cell, so treating them like a NiMH cell is pretty close to optimum anyway.

CP is both wrong and right, but "memory effect" is a slightly contentious subject anyway. In theory, there's no such thing; in practice, there is - but it's considered to be far less of a problem in NiMH cells than in NiCd.

My personal opinion though, as someone who's job for several years was the care and feeding of everything from little 0.15 A/Hr NiCd & NiMh cells up up 3200 A/Hr lead-acid cells: "Memory effect" is a very specific thing, and in 99% of cases where it's blamed it's not the real problem. Usually capacity loss is due to something else, like self-discharge (while in storage; IIRC, ~10% a month for NiCd depending on temperature), deep discharge (while in use), or overcharging (due to poorly-designed chargers) chemically and physically depleting the cell's capacity. Some of that depletion can sometimes be reversed by specific discharge / charge strategies. Repeated deep discharging followed by controlled high current recharging (with a high minimum current cutoff threshold) is a pretty good general strategy, and works quite well for most cases. Pulse or ripple charging can also 'restore' a depleted cell, but it depends on the exact problem (and a really good charger can determine that e.g. by testing cell impedance). Regardless of the method, you've got to be careful to not overcharge or overheat the cells.

OK, all that said … Timer-based chargers are crap. Most other consumer-level chargers are crap too. It sounds like you've got quite a good charger (and price isn't always a good guide). Discharging in a charge cycle is generally considered a naughty thing to do to NiMH cells, but it's sometimes necessary. And I hope that whatever your friend designed for cell maintenance in long-term storage didn't just leave the cells on float all the time, but let them self-discharge by 10~15% before topping them up ;-)
posted by Pinback at 5:32 PM on October 26, 2011 [1 favorite]


I would also note that the La Crosse charger shows you how much charge it put in each battery, and let's you set the rate at which your batteries charge. The cheapo ones generally just charge as fast as they can (which is not good for battery life or for getting maximum charge in to the battery). Anecdotaly, a friend of mine has seen the La Crosse charger successfully charge batteries other chargers have refused to (ie were 'dead').

That said, your rechargeable batteries will become less and less powerful over time no matter what.
posted by Phredward at 6:32 AM on October 27, 2011


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