Nickel batteries parallel charging

So the common wisdom says that charging Nickel (NiCd or NiMh) batteries in parallel is bad and would result in overcharging/thermal runaway.

But I just looked at my Rayovac PS3 charger and it does charge 2 AA or AAA in parallel. Opinions from NiCd/NiMh experts would be appreciated. Thanks.

The Rayovac PS3 is not a fast charger, btw. The charging rate is around 0.5A.

I had a massive fire and explosion from accidentally charging some NiMH cells in parallel, at a charge rate that was only about 0.1 or 0.2C, IIRC. I'd thought (wrongly) that at a low charge rate this would be safe, but it nearly burned my house down.

Are you sure that the charger in question really charges the cells in parallel? The reason I ask is that even with only 0.5A of charge current that sounds pretty iffy to me. I was much higher capacity cells charging at just 0.75A when my fire occurred: http://endless-sphere.com/forums/viewtopic.php?f=14&t=3241&p=47328#p47328

Yes I'm absolutely positive. I have opened it up and verified it.

I have been using this charger for 10 years. Now that I discover its "parallel charging" "feature", I'm beginning to wonder how many AA and AAA I have killed during that time.

Jeremy Harris said:

I had a massive fire and explosion from accidentally charging some NiMH cells in parallel, at a charge rate that was only about 0.1 or 0.2C, IIRC. I'd thought (wrongly) that at a low charge rate this would be safe, but it nearly burned my house down.

Click to expand...

There is a difference. You were charging at least 2 strings in parallel. Each string has at least 20 cells in series.
The most this charger does is 1s2p at around 0.5A.

SamTexas said:

Jeremy Harris said:

I had a massive fire and explosion from accidentally charging some NiMH cells in parallel, at a charge rate that was only about 0.1 or 0.2C, IIRC. I'd thought (wrongly) that at a low charge rate this would be safe, but it nearly burned my house down.

Click to expand...

There is a difference. You were charging at least 2 strings in parallel. Each string has at least 20 cells in series.
The most this charger does is 1s2p at around 0.5A.

Click to expand...

Yes, but in my case it only took one cell to fail to cause the fire, so in essence the cause is similar.

My guess is that your charger may have been designed back when NiCd cells were far more prevalent than NiMH, and it's the latter than are more sensitive, it seems, to being over-charged. My experience with NiCd's is that they were remarkably robust and seemed to accept an awful lot of charge and discharge abuse without any noticeable effect on performance.

It seems extremely unlikely this charger is actually charging them in parallel if it's truly charging them to 100%. If it's only charging them to ~80%, you could safely charge them in parallel, as you just terminate charge the moment voltage starts to drop.

If I was setting up/designing a nickel based charger, I would be pretty tempted to skip the last 20% SOC where the voltage drops as it continues to charge. Otherwise, if you install one cell at 80% charged in that charger with a cell at say 5% charged, and begin charging, the 80% cell is going to be fully charged when the 5% cell reaches about 50%, and you either can't continue to charge the 50% cell, or risk exploding the fully charged cell, either way it's a pretty crappy outcome.

liveforphysics said:

It seems extremely unlikely this charger is actually charging them in parallel if it's truly charging them to 100%. If it's only charging them to ~80%, you could safely charge them in parallel, as you just terminate charge the moment voltage starts to drop.

Click to expand...

That's an interesting and reasonable observation. Quite straight forward to prove or disprove too.

Next question for the Nickel batteries experts: Which single (or multiple single) cell charger do you recommend? One that you have personal experience with that actually fully charges a NiCd or NiMh battery. Thanks.

So far I haven't found one I can actually recommend, as all of the ones I've used that are made for charging individual-type cells (AA, AAA) have either damaged cells (possibly by parallel charging them; never opened them up to check, but I am going to now), or the chargers failed and stopped recognizing batteries at all.

This includes one by Sony, two by Olympus (for their old AA-based cameras), two generics, one Ray-O-Vac, one Realistic, and a few others I don't remember the names of (that I dont' have anymore).

I have tried using a Tenergy NiXX charger that's for 7.2V-12V packs to do charging of some of my AA and AAA flashlight batteries, and it appears to do ok most of the time, but I usually have to discharge some fo the cells manually beforehand so they match voltages (although probably not SoC), so that they all charge more or less correctly together. I have to put the cells next to the temperature sensor as that charger terminates charge partly by Delta-T, I think. I'll have to dig out that charger to get it's model.

I have also tried charging some on the Accucel6, witout a thermal sensor, and it appears to correctly terminate charge via delta-V, but I have not extensively tested it.

I tried testing using the Venom, but it wouldnt' start charge without the thermal sensor, and I didn't have one at the time and never went back to try again.



List of various previous discussions of paralleling Ni chemistries.
http://www.endless-sphere.com/w/index.php/Batteries_NiMH_%26_NiCd

I just read this a Prius forum.

I have 120,000+ miles on my 2004 Prius with 90,000 of those miles on my 18Ah parallel system. Both the OEM HV Battery & the parallel auxiliary array test with internal resistances that are well within new specs. I would wager that most Prius with 120,000 miles do not have an OEM internal resistance that is as healthy as mine is.

Read more: http://priuschat.com/threads/parallel-battery-packs.21993/#ixzz2I55sXfh2

Click to expand...

May be the Prius never really fully charges its NiMh battery to get more life out of it.

as long as the cells are never brougth to the point of delta-v, you could parallel charege them safely enough...but you'd also eventually need a separate balancing system to manually discharge each cell to equalize them because you wouldn't be doing it the way the chemistry usualay does.

Asl ong as the cells are healthy and new they'd be reltatively equal capacity/ir/etc, but afgter aging they're like any other--they dont' reach "full" at the same time so to use the full capacity of the cells you may ahve to drain off the high cells before they reach delta-t. Assuming cell=levelparalleling, it could be problematic if paralleled cells became aged in different ways, and if you didn't know it....


if paraleled at stick level only, then as long as the aging of celsl on average makes ech stick about he same, it also should be ok...but I doubt they'd age taht way. so you might have to cut usable capacity by a fair bit to ensure you don't reverse a cell. or else monitor every cell for lvc and hvc, etc.


I dunno...i guess it's easily possible to do a stick-level parallel, with the lvc and hvc, or using individual cell-level charging instead of bulk charging, but that all gets so complex it kinda negates teh simplicity of normal NiCd/NiMH bulk charging.

I believe it is true, that the prius battery never fully charges. I can't remember the numbers exactly, but it was something like cycles that had 50% dod at the bottom and 80% at the top. Whatever the real numbers are, I was amazed at how conservative a cycle it was.

Re cheap nicad chargers. My charger works on pairs of cells at a time. But the charging light does not come on untill the second battery is inserted. I always interpereted that to mean that two cells were charged in series, since one cell cannot complete the circuit to start the charging.

If paralell, would not the charger turn on when one battery was inserted?

I think my little nicad nimh chargers do a 2s charge. Not a 2p charge.

Even more amazing is how heavy that NiMh battery is, considering its tiny 1,400Wh capacity.

I have those chargers that require two cells to charge too. I have verified that the pair are charged in series. But they also kill the batteries if they are not at about the same SOC or if the are not of the same capacity. Hard to believe that these products have been around for so long.

Yep, that is true. I mark my batteries in pairs, so I don't mix em up. Don't screw up and put a charged one in with a discharged one!!! If you do, the charged one will get hot very quick.