How can I discharge NIMH battery safely?

[watdahel]

I'm trying to get the most out of my NIMH batteries by discharging them completely. One of the cells has more charge than the others so I want to make sure this particular cell has the same charge as the others before I recharge them. Is it safe to short the batteries by directly connecting the +/- terminals together with a wire? I figured I would discharge them to 0 volts or close to it.

I don't know much about the specs of my charger so I don't know how it determines the batteries are fully charged. I do find that one of the batteries gets hotter than the others so I figured it might be overcharging this particular cell while the others aren't getting fully recharged.

I wish I had a Maha C401FS charger. That thing charges 4 batteries independent of each other so even though the batteries have different charges, the charger will only recharge each of them until they are full.

 

[crypticlogin]

Is it safe to short the batteries by directly connecting the +/- terminals together with a wire?

Just about any decent battery will tell you not to do exactly that... so don't. If you really need to (near) fully discharge them, use a resistor or a small lamp so you've got a load on the batteries.

I do find that one of the batteries gets hotter than the others so I figured it might be overcharging this particular cell while the others aren't getting fully recharged.

Why don't you just take the hot cell out and let the others charge for a bit longer?

 

[corkyg]

You can get a Maha C204F charger with EIGHT 1800 NiMH batteries for $39.95. I have used that charger for two years and it works perfectly. All my AA NiMH batteries are kept in top condition. The new C402FS is simply a lot faster - but speed is not a factor for me when I condition my AA batteries for my digicams.

 

[DJFuji]

i thought NiMH batteries don;t have memory like NiCds?

 

[stevewm]

Originally posted by: DJ Fuji
i thought NiMH batteries don;t have memory like NiCds?

NiMh batteries can develop the memory effect, though it is not nearly as bad as NiCads. And it is usually very easy to fix. Its a good idea for general upkeep to discharge the batteries about every 10-15 cycles.

Though using the batteries until your device complains of a low battery is enough of a discharge, it would be a bad idea to discharge it any futher. Many higher end NiMh chargers have a conditioning mode that will discharge the battery to the proper level before recharging.

 

[watdahel]

From my understanding, both NIMH and NICAD batteries are susceptible to the so called "memory effect" disease; only NICAD batteries are more prone to it since they are old-technology. The "memory effect" occurs when batteries are overcharged and batteries overheat. This is exacerbated when you recharge a half-full battery and the charger doesn't or can't properly detect when a battery is fully charged and simply charges according to a built-in timer.

That's all I know. I'm tired of learning any further. Who wants to add something?

 

[stevewm]

Actually memory effect is not caused by overheating/overcharging. Overheating and overcharging results in a ruined and un-fixable battery, not memory effect.

Memory effect comes from recharging a battery when it is not completely discharged. NiCad will get the memory effect nearly every time you do this to them. To cause memory effect in a NiMh battey you would have to put it through several "half charge" cycles. Usually a single full discharge and recharge cycle will restore a NiMh battery, while for NiCad batteries it may take SEVERAL cycles to fully restore them.

As I said before, use your NiMh batteries until your device complains of low power, preferably to the point where the device actually shuts down because the batteries are so low. At this point they should be very close to 1 volt and therefore dead/discharged.

To answer one of your original questions: You should NEVER discharge a NiCad or NiMH battery below 1 volt. Doing so will either ruin the battery (it could cause the polarity to reverse, once that happens the battery is ruined!) or significantly shorten its useful life. When a NiCad or NiMh battery is at 1 volt, it should be recharged ASAP to prevent damage. Simply shorting out a battery with wire is a very bad idea. You want some load. Its best to get a charger that has a "condition" mode. In this mode a charger will discharge the battery to 1 volt before recharging it. Failing that hookup a lightbulb to the battery along with a volt meter, when it reaches 1 volt the battery is discharged.

 

[watdahel]

Well, I don't believe the effect of reversing polarity if a battery is left uncharged once it's at 1 volt; afterall, these batteries hang on store shelves for months and they don't die just because they're not being recharged.

1 point to me.

 

[TheCorm]

Quote

--------------------------------------------------------------------------------
Is it safe to short the batteries by directly connecting the +/- terminals together with a wire?
--------------------------------------------------------------------------------


Just about any decent battery will tell you not to do exactly that... so don't. If you really need to (near) fully discharge them, use a resistor or a small lamp so you've got a load on the batteries.

Doesn't this cause the battery to heat up and then even explode?

 

[stevewm]

Originally posted by: TheCorm

Quote

--------------------------------------------------------------------------------
Is it safe to short the batteries by directly connecting the +/- terminals together with a wire?
--------------------------------------------------------------------------------


Just about any decent battery will tell you not to do exactly that... so don't. If you really need to (near) fully discharge them, use a resistor or a small lamp so you've got a load on the batteries.

Doesn't this cause the battery to heat up and then even explode?

The whole point of having a load on the battery is to prevent this from happening. A high value resistor or lightbulb will provide enough load to prevent the battery from heating up while discharging it at the same time


erwin1978: ACTIVELY discharging a battery below 1 volt will usually cause the effects I mentioned. And if you look at the voltage of a battery that has been sitting on the store shelf for months you'll find it is above 1 volt.... NiMh or NiCad batteries won't really self discharge below 1 volt.....