Charging 3 eneloops in series as lithium ion?

I have a 3xAAA powered headlamp that I use 3 nimh batteries in. I have several different chargers, but my usb powered charger (Lii-202) only holds 2 batteries at one time. This is the charger I keep in my vehicle.

Is there any harm in placing those 3 nimh batteries in series in a battery holder from a cheap 3AAA flashlight and charging the batteries in series as lithium ion cell? These cells should all have relatively the same amount of charge and have always been used together, I try to group my batteries and use the same ones together when possible.

My thinking is that charging the cells to 4.2 volts in series would actually be slightly undercharging the cells at ~1.4v each, which, in theory should actually help prolong the life of the batteries.

Are there any other problems I need to be aware of with charging nimh batteries in this way?

Obviously I could just charge the cells individually, but I'm just exploring charging the cells in series as a possibility.

Let's hear your thoughts!

There is nothing in principle against it. But it will take longer than charging with -dV and you have to build an adapter.
However, if one battery gets empty sooner than the others one day, this method of charging can (probably) not match it.

Hello Twicefuzed,

You can get away with that for a few cycles but eventually the cells will get out of balance and then performance and capacity will drop off. Eventually one cell will end up over discharged and could end up damaged.

An alternative would be to do individual cell charging every few cycles to bring all the cells up to an even state of charge.

Tom

This will work fine if the cells all have the same capacity and state of charge at the start. However, if one cell is more charged or has less capacity, it will reach full charge before the others. This one will be overcharged while the others are still charging. This will damage or destroy that cell.

On the other hand, as SilverFox points out, if charging stops before one cell is fully charged, that cell may get overdischarged when you run your light. Personally, I think this is less destructive if you charge the cells soon afterward, but YMMV.

Personally, I wouldn't recommend this unless you know your cells well!

TwiceFuzed said:

Is there any harm in placing those 3 nimh batteries in series in a battery holder from a cheap 3AAA flashlight and charging the batteries in series as lithium ion cell? These cells should all have relatively the same amount of charge...

Click to expand...

I see no harm, but according to this paper the cells will stay undercharged more than practical. You will get only 34% of capacity (760mAh instead of 2200mAh) from a 3S NiMH pack when charged on a LiION charger at 1C rate (2300mA with 230mA termination).

But charging at lower rate (500mA with 50mA termination) could help. Please give it a try and report back your findings.

Cheers,
AA Cycler

I was making custom device that runs on 1xAA (well...2AAs in parallel for capacity reasons) and I wanted to add charging via micro-USB (so the device can run on external power source...and well...it would be strange to have external power connector to just RUN the device and not CHARGE, right?

I made simple 1.40V (if I remember correctly) constant current/constant voltage circuit and it runs perfectly. Batteries can be connected indefinitely on this voltage withnout any noticable problems. Thay are charging with 200mA limit. After they reach target voltage, the current lowers itself and settles to few mA after long period.
The problems with this charging method:
- this probably won't work reliable over wide temperature range
- you can charge only at low curernt
- you won't reach 100% capacity - as far as I remember, after 2 days the batteries had around 80% capacity when discharged.

czAtlantis said:

I made simple 1.40V (if I remember correctly) constant current/constant voltage circuit and it runs perfectly. Batteries can be connected indefinitely on this voltage withnout any noticable problems. Thay are charging with 200mA limit. After they reach target voltage, the current lowers itself and settles to few mA after long period.
The problems with this charging method:
- this probably won't work reliable over wide temperature range
- you can charge only at low curernt
- you won't reach 100% capacity - as far as I remember, after 2 days the batteries had around 80% capacity when discharged.

Click to expand...

That makes sense. NiMH tend to terminate at around 1.55v when charging with the -dV/dt method. So, charging to only 1.4v will definitely reduce the charge level. If the OP does something similar to you, he should get similar results (3 x 1.4v = 4.2v). Only issue would be keeping the cells somewhat balanced, but using well-matched Eneloops should help.

I was under the impression the two different battery chemistries between Nimh and Li ion would use different charging algorithms schemes and two different termination techniques. But I could be wrong or maybe it's no big deal.

No, you're not wrong. It's just that if you're not fussy, the LiIon algorithm can do a passable job charging 3 NiMH cells. The reverse is absolutely not true.