Nickel battery balancer

Question

TLDR: The question is about retrofitting a balancer to the Ni-MH battery of an existing appliance (that doesn't trickle charge the battery), preferably in the way that will not require frequent access to the battery. Changing the charge/discharge logic of the appliance doesn't look feasible - this way it would be really better to rework the circuit for Li battery.

While there's a lot of Lithium balancers for every common Li chemistry, Nickel (Ni-MH or Ni-Cd) balancers seems to be nearly non-existent.

I know it's told that Nickel cells would balance if charged with a standard C/10 charge for 16 hours. And in pre-Li era, enthusiasts would make the same C/10 "forming charge" of the pack after assembly, then use it as is, without caring too much about disbalance.

But recently I've run into a situation where it's desirable to have a Ni-MH balancer.

The device (robotic vacuum cleaner) charges 12S Ni-MH 2200mAH battery with ~C/2 current, with dV/dt and dT/dt end-of-charge controls seemingly in place. (And discharges them at ~C/2 when running.)

After few hundred cycles (a year or so of nearly everyday use) the usable capacity of the battery significantly drops.

Cell-by-cell test clearly shows some cells where chronically undercharged, that resulted in polarity inversion during discharge and rapid deterioration of affected cells.

I think that proper balancing could significantly (2 times or so) prolong overall usable battery life (as the healthy cells in the battery still have 80%+ of their initial capacity).

Searching the net resulted in few proposals for Nickel balancer:

1) Very practical and simple voltage limiter: https://www.electroschematics.com/balancing-ni-mh-battery-packs/

A pair of rectifier diodes per cell that will partially shunt the cell once it's nearly full, thus limiting it's charge, and giving runners-up a chance to catch up.

Requires all diodes to be on a common heatsink for temperature equalization, will drain batteries fast if not disconnected, will limit usable capacity of the battery.

While cruder, it looks to be OK for occasional balancing, but it's impossible to integrate such a "balancer" into the battery.

2) Microcontroller-based inductive balancer: http://cache.freescale.com/files/32bit/doc/app_note/AN4428.pdf

Good all-around solution, but it looks like an overkill for the case. The feasibility criteria is the cost of the balancer that should not significantly exceed the cost of cells for a new battery.

3) Simple charge shuttle balancer: https://easyeda.com/Popov_Alex_r/Battery_Balancer_Ni_Cd_Active1-840522a2a4e44fe89bf70d560e4607f9

Looks great for integration with standby current of 80uA (that's 60mAh/month, comparable with self-discharge of 2200mAh Ni-MH cells), but charge shuttles are not particularly effective, and I'm in doubt if it would be able to reach balanced state on a used battery with maximum balancing current of just 10mA.

Looks like none of purpose-built balancing ICs are able to work with Nickel cell voltages, and most are kind of specialized on Li cells with undervoltage protection on ~2.75V.

Any other ideas on Nickel battery balancer that could be integrated into the battery of an existing appliance?

Answer 1

Active balancing is simply not necessary with NiCd and NiMH battery chemistries. It doesn't harm a fully charged cell to be somewhat over-charged in order to bring a slower-charging cell up to full charge.

You can't do that with rechargeable Lithium cells — they tend to do bad things with the slightest amount of overcharge, which is why active balancers were developed.

Answer 2

The problem you obverved is very common in large stacks of Nickel batteries: Some cells get polarity inversion every cycle, and rapidly detoriate, so the whole pack dies. But the primary reason for this effect is not (in my oppinion) misssing balancing during charging. The charging efficiency drops as the cell gets full, as it starts to convert input energy to heat. A cell of the same capacity that had less charge at the beginning of the cycle will be able to store more of the energy applied to it than a cell that gets (sightly over-) full during the charging cycle.

The typical problem that destroys the Nickel Packs is cell diversity: Some cells have less capacity and experience reversal on every deep discharge and also get overcharged in the following charge cycle (dV/dt does not trigger if a single first cell gets full...). Both the reversal and the overcharge make the cells lose even more capacity until the cells fail. Another source of diversity of the state-of-charge is differences in self-discharge. But also in this case, they get rebalanced in the next charge cycle (and if you cycle your cells every two days, self-discharge is no problem in non-defective quality cells).

To avoid the early failure of the pack, you need better end-of-discharge protection that detects a reversed cell even in the case that the other 11 cells are still delivering fine. There are different ways to implement this:

• You could make the end-of-discharge voltage 11*1.25V = 13.75V, so one cell at zero (not yet reversed) while 11 cells at a typical 1.25V would already trigger the reversal protection. But this usually causes way too early discharge termination, because the voltage also drops on current spikes (you would have to compensate for that), and you can safely discharge the cells to 12*1.00V if they are "balanced", so the typical cut-off voltage is 12V for a 12S pack.

• You could detect a rapid voltage drop by around 1V as a sign of cell dropping out (again you hav to compensate for current spikes to avoid mis-triggering).

• You could monitor the voltage of individual cells, or smaller substacks (so you cut out if any of the 4 sub-stacks of 3 cells each drops below 3 volts).

All of these possiblities increase cost and complexity of the discharge circuit, as does a good selection of nearly equal cells for the pack to begin with, so device manufacturers often take the cheap and easy way.

And finally an answer to your question: To balance the pack, just charge the pack for 3 hours at C/10 after the proper dV/dt or dT/dt termination (if your device doesn't do trickle charging or top-off charging) every other month. Your question correctly mentioned the balancing effect (the full cells get overcharged by a safe amount, any non-full cells get topped off). For the reasons explained above, I do not expect this to provide a significant extension of pack life, though.

Answer 3

I'm not sure what they're called, but the simple LiPo balancer circuits I've seen just shunt the cell current when it gets up to 4.2V.

I'm not sure how that would work with NiMH, though, because they tend to be self-limiting in voltage (IIRC, NiMH voltage actually goes down a bit when they're at full charge). So you're balancing for different reasons: with LiPo charging, you're avoiding overcharging, which destroys the cells; with NiMH charging you want to avoid undercharging.

If you could jigger the charging circuit to charge the battery nearly full, then drop to a C/10 trickle charge, that may be sufficient.