Can you mix rechargeable batteries with different mAh ratings?
January 16, 2006 8:22 PM   Subscribe

Can you mix rechargeable batteries with different mAh ratings?

I only use Energizer NiMH rechargeable batteries. All of my older batteries are rated at 1850mAh and I bought a bunch of new ones that are all rated at 2500mAh. Can I mix these two batteries with each other in the devices I use? Can I charge them together or should I keep them all separate?

The reason I ask is that it is often stated that you should never mix fresh batteries with half dead batteries. Is mixing the two different ratings equivalent to that since the lower rating batteries will die sooner?
posted by monsta coty scott to Technology (9 answers total) 3 users marked this as a favorite
Amp-Hours are a rating of how much energy a battery holds. A theoretical 10 amp-hour battery could deliver 10 amps for an hour, or 1 amp for 10 hours, or half an amp for two hours, and so on. Now, there are limitations due to the internal resistance of the battery (and probably some other things I don't know about), so you can't extend the current draw to absurd values.

So what does all of that mean in the context of your question? If you have two batteries of different amp-hour ratings, one will run out before the other. This means one will be providing all the power after a certain time. If they are connected in series it will be drawing it through the other battery, which may be a bad thing, but I'm not sure. If they are in parrallel it will still be drawing all the power from the longer lasting battery, but won't affect the other(s) at all.

The problem probably will come if you have a few batteries in one device and they all have low ratings except one. Then at some point that one will have to provide all the power, adversly affecting it's performance.

posted by phrontist at 8:31 PM on January 16, 2006

It will definitely reduce the effective capacity of the set as a whole, perhaps not quite to the lower of the two ratings, but very close. You will also be over-dischargeing the lower capacity battery, which isn't good.

A similar argument applies to charging. Overcharging a cell will overheat it, reducing its useful life. On rare occasions it might 'explode', which really means gases escaping from safety vents.

I don't see why you would want to mix them. Best case you are voluntarily reducing the capacity of your higher capacity cells... I'm sure there are arguments against mixing that I haven't discovered... On the other hand, the grave warnings make it seem like a life or death issue, I'm sure it isn't that serious.
posted by Chuckles at 9:30 PM on January 16, 2006

posted by Mr T at 10:21 PM on January 16, 2006

"Over-discharging" is kind of an understatement; what happens is that eventually the higher-rated batter starts trying to charge the lower-rated battery in reverse. This quickly kills the lower-rated battery, often permanently.

If you don't discharge the batteries very far, then it won't be a problem. But if you do discharge them fully (like most of us do; we don't change them out until they can't power the device any more) then the batteries should ideally be not just the same capacity, but also equally well-charged to start with, about the same age, etc., etc..
posted by hattifattener at 11:12 PM on January 16, 2006

What everyone else said - it's a good habit to keep batteries in 'sets' through their lifetime (elastic bands are handy here). Charge and recharge the same batteries of the same type and spec together at the same time and use them in the same devices whenever possible. Batteries used together should be of the same rating, and the same brand. Manufacturers have been known to be somewhat 'flexible' in their self-rating (for marketing purposes) - cells of different brand with the same numbers on the side might not be as alike as we might want.
posted by normy at 12:00 AM on January 17, 2006

I use a Sharpie to write a unique number on pairs of batteries to keep my NiMHs in sets.
posted by zsazsa at 5:30 AM on January 17, 2006

Theoretically safe, so long as none of the cells reach discharge. In real life, best performance is given, as mentioned, by matching a set and keeping it together for the set's working life.

This would be dramatically dangerous with Lithium Ion batteries, which have this failure mode called "catch fire." However, becuase of that, you only see Li-Ion batteries in prebuilt packs, with a charge/discharge controlling circuit onboard the pack to prevent abuse.

Personally, I've mixed in emergencies, and it worked, but battery life wasn't good (the weak cell drains, and drops the pack voltage down to the point where the device starts flaking out.)

So: If you must, you can, but be gentle with them and expect shorter charge life, and shorter battery life. Best to buy a couple of new sets and keep them as batteries of (err) batteries.

(Tech note: A single cell is tied to other cells to form a battery. 1.5V Alkaline, 1.3V NiMH and 1.2V NiCads are cells. 9V Alkalines are batteries, they're 6 1.5V cells in one pack.)

(English note: Usage calls them batteries, regardless.)
posted by eriko at 5:42 AM on January 17, 2006

Do not do this, assuming the cells are in series, which they almost certainly are. The lower capacity ones will discharge first and then you will be impressing the voltage of the remaining cells across the load and the discharged cells.

That means you're reversing the voltage on the empty cells, which is invariably bad. NiCd cells will outgas (burst), NiMH probably the same, Li-Ion will catch fire. It's exactly the same as charging them backwards and probably at a much higher current than a charger would use, even in the correct direction.

So yes, it will work for a short time. Then you will destroy your cells, even though the device may appear to continue functioning since there is sufficient voltage from the remaining cells. Gassing/leaking cells may leak nasty fluids into your precious electronics, making it a terminal event for more than just the cells.

This is the same reason you should always charge and use your cells as a set - if you mix fresh & partially-discharged, the full ones will destroy the discharged ones. For this reason, you should always use a smart (negative delta-V, not timer) charger with individual cell monitoring that will ensure they're all full. Just because you haven't used a cell since charging doesn't mean it's properly charged, either. NiMH lose charge rapidly while sitting on the shelf - tens of % in a couple of weeks. So keep your batteries charged and if they're left for a month, do a discharge/recharge cycle.

For the same reason, do not mix cells of differing ages though they may be of the same capacity. As they age, the capacity reduces and you get the same effect.
posted by polyglot at 6:37 AM on January 19, 2006

that should be "discharge/recharge the day before you use them".
posted by polyglot at 6:39 AM on January 19, 2006

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