Electronics, Lighting, & Gadgets - How do you refresh or recondition batteries?

I've got some Sanyo 2700 NiMH AA batteries that seem to be holding their charge less and less. I've had them for 2+ years and use them in my commuting light 3-5 times/week. How do I refresh or recondition the batteries? My charger is a "microprocessor controlled ultra-fast" model that I bought from one of the one-line battery stores, batteriesamerica.com

http://www.batteriesamerica.com/new_page_2.htm

I've got some Sanyo 2700 NiMH AA batteries that seem to be holding their charge less and less. I've had them for 2+ years and use them in my commuting light 3-5 times/week. How do I refresh or recondition the batteries? My charger is a "microprocessor controlled ultra-fast" model that I bought from one of the one-line battery stores, batteriesamerica.com

http://www.batteriesamerica.com/new_page_2.htm

My charger has the option to draw the batteries all the way down (discharge) then recharge them. It is a much longer recharge cycle when you do that. Suppose to keep the "memory" issue minimized.

http://www.tenergybattery.com/index.php?page=shop.product_details&flypage=shop.flypage&product_id=53&category_id=15&manufacturer_id=0&option=com_virtuemart&Itemid=27

I bought this charger and 20 2300 NiMH AA, dozen or so D and C batteries three or four years ago. It has been a great investment.

Look for a charger that specifically states that it can do battery conditioning. Here is one from Battery Junction.

http://www.batteryjunction.com/8800.html

The vanson charger also has a conditioning mode

http://www.batteryjunction.com/vabcunsmch.html

Another good clue is, "don't use an ultra fast charger". I won't use any charger that charges a AA from empty in less than about 3 hours (base rate of about an amp). It's just not good for the cells.

If the cells are getting uncomfortably warm, seriously, can that be good for them?

Have you been utilizing partial capacity and recharging them daily and leaving them on trickle charge? Since your charge does not have a conditioning mode you might try running them all the way down before recharge. It is generally recommended to charge at a high rate for the majority of the charge (I believe 80 or 90 percent) and then complete the charge at a slow rate when resistence is highest. It sounds like your charger is designed to do this so the periodic full discharge portion may be what you are lacking.

Nicads are recommended to be fully discharged regularly and Nimh's periodically fully discharged to retain maximum capacity. I think the fast/slow charge strategy is recommended for both.

It seems like it should be easier to get information on maximizing rechargeable battery cycles but this does not seem to be the case.

The vanson charger also has a conditioning mode

http://www.batteryjunction.com/vabcunsmch.html

I suspect this charger is often branded differently. Looks the same as my Tenergy charger.

Another good clue is, "don't use an ultra fast charger". I won't use any charger that charges a AA from empty in less than about 3 hours (base rate of about an amp). It's just not good for the cells.

If the cells are getting uncomfortably warm, seriously, can that be good for them?

+1 on Fast chargers, many of them are very hard on nimh cells and you will not get the longevity from a better charger..

To refresh the cells, the best thing to do is discharge them until they reach about 1.2V and then charge them. You may need to repeat this a few times.

There is some misinformation in this thread.

Ideal charge is at rate is 0.5C to 1C meaning 50% to 100% of capacity in one hour -- most good chargers can do this. At these rates, charge will take 90 min to 3 hours. A minimum of 0.33C is recommended for good cells. A good charger should be able to monitor the cell temperature and cut charge if it overheats, monitor for -ΔV as an indication of a full charge, and have individual charge circuits (no need to charge in pairs).

Slower and less expensive chargers are sometimes timer based rather than -ΔV cut meaning that the charger may not charge the cells to capacity.

Also keep in mind that NiMH cells have limited lives and are consumable items even though they can be recharged. Charge-discharge cycles introduce wear into the cell and over time the capacity of the cell will diminish. Nothing you can do about it because this is a property of the cells.

NiMH tend to self-discharge at a rate of about 30% per month so keep that in mind. A recent (two or three years) introduction are low self-discharge NiMH cells. These are supposed to be able to retain about 90% of their charge after a year or so. The most popular ones are Sanyo Eneloop. These can be found in a kit at Costco with a timer based overnight two circuit charger. The cells are great, but the charger leaves much to be desired. Even though these have (relatively) lower capacity compared to 2700 mAH, it probably won't make much of a difference because you need to keep in mind the self-discharge rate of that 2700mAH cell is far higher.

Arguably, the two best AA-AAA chargers out there are the Maha MH-C9000 and the Lacrosse BC-9009 or BC-900. These are both -ΔV cut chargers, monitor temperatures, and have individual charge circuits. Additionally, they have features to charge, discharge, refresh, and test cells. The MH-C9000 has more features and is bigger and costs more. Tthe Lacrosse should work well also, but the default charge rate is lower than it should be for most AA NiMH applications.

+1+1+1 to most of erikcam's reply

To 'refresh' them you want to discharge to 1V but not below (1.2V is their nominal full charge). Put them in a flashlight and let them burn (dont use your bike light as it may depend on air flow for cooling - you didnt mention the light). When it dims, check the voltage. If it is >1V, let them burn some more. Some LED flashlights turn off at 1V, others dim. When you get to 1V, charge them up, then let them rest 1 hr. Repeat 5 or more times.

This mainly acts to remove any 'memory effect' and excercise the cell. You might only get a 5% increase in capacity, or you could get more. Of course, without an analyzer you wont know if it is doing any good.

High cap cells (>2500mAh) are more suspectible to aging faster and other things like damage from overcharging, overheating, fast discharging and dropping etc etc etc. If/when things start to go bad it can begin to cascade. Regular cells (2000-2500mAh) use a thicker separator which makes them more durable at the expense of capacity. There is no free lunch.

Three years at 3/week is 450+ cycles. Can you really say you didnt get your money's worth? I'd retire them from bike light usage and relegate them to blinkies, flashlights, back up status or in a remote. The marketing may say they are good for 1000 cycles, but your cells could be more aged as a result of the charger, charge methods and/or how they are used.

Hard to tell about the charger. It says it uses different rates for AA and AAA (good) but doesnt say what the rate is. The timer back up is good, 'ultra fast' is not. It also doesnt say if it channels are independant, but that might be meant by 'SMART'. If the cells comes off the charger very very warm, get a new one. If they are nearly hot on the outside, imagine how toasty it is inside.

Only you can decide if a 'deluxe' charger is worth the $40-$90. They do take better care of cells and quantify things like actual capacity so you can group cells with similar capacity for max performance. Most have a default settings that you dont have to worry about -dV or .5C vs 1C, though they tend to be quite conservative that way. A decent charger with independant channels, multiple charge cutoff methods are only $5 or $10 less than ones with analysis capabilities.

HTH

Has anyone actually "revived" AA cells that have lost ~50% of their capacity? I have never had any success, even using the "refresh" cycle of my chargers (I have more than one kind). My batteries last about a year before I need to replace them.

I throw them away at 70% of their new capacity, so I havent tried from 50%. I did just revive a 3 or 4 yr old AAA cell from 70% back to the ~88% of his brothers. It took a while though.

Refreshing is mainly to minimize the effect of 'memory'. It doesnt do much for a cell with other problems. It seems more effective early on to bump its capacity 5-15% when it has sunk to 80 or 90%. The more the capacity diminishes, the more likely that a greater portion of the damage is permanent.

It seems like a lot of that happens when we perceive the cells to be too new to check out. Which is prolly why they say to exercise the cell every 1-2 months (ie a full disharge, not just use it).

A cell at 50% seems like a lost cause, though.

+1+1+1 to most of erikcam's reply

Hard to tell about the charger. It says it uses different rates for AA and AAA (good) but doesnt say what the rate is. The timer back up is good, 'ultra fast' is not. It also doesnt say if it channels are independant, but that might be meant by 'SMART'. If the cells comes off the charger very very warm, get a new one. If they are nearly hot on the outside, imagine how toasty it is inside.

HTH

There is a slide switch on the face to select 2 or 4. I assume this charger only has two charging circuits for either 2 or 4 cells. I don't see any other purpose for such a switch.

Has anyone actually "revived" AA cells that have lost ~50% of their capacity? I have never had any success, even using the "refresh" cycle of my chargers (I have more than one kind). My batteries last about a year before I need to replace them.

I have not. Nor have I seen memory effect from NiMH cells.