unprotected Li-ion in series a hazard, but NiMH not?

Ok, I'm aware that unprotected Li-ions in a series represents an alleged risk of explosion.

Queries -

1 - in simple terms that even Powernoodle can grasp, why? Battery A conks, and Battery B starts to charge it, resulting in heat, pressure and boom?

2 - is the same not true with NiMH batteries, and if not, why not? Different chemistry not susceptible, or a voltage thing, or what?

I know there are threads out there on the first question, but the search function isn't up to speed yet.

I'm basically trying to figure out why 9 NiMH batteries in series in a Mag85 is safe, whereas 2 R123s in series isn't.

thanks!

Hello Powernoodle,

First of all, let me point out that all batteries have some risk because they store energy.

There have been a few reports of NiMh cells exploding when hot cells were recharged at very high rates. The explosion resulted in spilled corrosive chemicals and shrapnel flying across the area. This is known as "rapid venting."

Li-Ion cells operate at a higher voltage, couple that with low internal resistance and you have the ability to deliver high current which can heat the cell up. When internal pressure increases the vent is suppose to relieve it, but if it does not operate properly, or if the pressure builds faster than it can relieve it, you end up with a bad situation.

Remember in the case of NiMh cells, when things heat up and pressure builds you end up with "rapid venting," in the case of Li-Ion cells you end up with "rapid venting with FLAME."

It's bad enough to have hot chemical spilling out and shrapnel flying, but now we are adding fire to the mix.

There is nothing wrong with using Li-Ion cells in series, as long as you don't abuse them. Abuse comes in the form of improper charging, drawing more current than they can safely provide, and over discharging. The penalty for abusing NiMh cells is usually a dead cell or a bit if a chemical spill. The penalty for abusing an Li-Ion cell can be fire.

Tom

1 - Battery A conks, and Battery B starts to reverse-charge it, resulting in heat, pressure and boom. Its the reverse-charging part that causes it to heat-up and increase internal pressure, fast.

2 - AFAIK, with Li-Ion's, the flame and boom part has something to do with its chemistry. NiMH's only have corrosive electrolyte whereas Li-Ion's have corrosive and flammable electrolyte.