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After several of searches on the internet for information about charging NIMH batteriers, i feel the need for some clarification. I've gotten my hands on a bunch of battery packs. Each battery pack is 24V 1500mah, they are built on 20 x 1.2V 1500mah GP AA batteries.

My goal is to parallel connect 10 packs achieving 24v 15A. Until recently i thought this was easy and safe.

questions:

  1. Is it safe to connect and discharge 10 packs in parallel?
  2. How is it possible to charge all 10 packs at the same time? (wall plug)
  3. Is it possible to use a 24V PMM solar panel
  4. Do i need a cutoff when voltage is getting low?

Thanks in advance :)

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  • \$\begingroup\$ What does 'PMM' mean? \$\endgroup\$ Commented Apr 1, 2020 at 17:58

2 Answers 2

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1) Is it safe to connect and discharge 10 packs in parallel?

Yes. However, make sure they have the same voltage before you connect them in parallel. Either charge/discharge to the same voltage, or connect them with a resistor to limit the current while they equalise, before connecting properly.

Given the high current capability of NiMH, it would be very wise to include a fuse in series with each battery. If one pack suffered a short in some of the cells, the other 9 could gang up on it to drive a fire-starting current through it.

2) How is it possible to charge all 10 packs at the same time? (wall plug)

Apply a charging current from some source.

3) Is it possible to use a 24V PMM solar panel

That's the nominal voltage of the batteries, you'll need more than 24v to drive current through them.

4) Do i need a cutoff when voltage is getting low?

While individual NiMH cells don't get damaged even if abused down to 0v, a battery of series connected cells can reverse any single cell that discharges first, so sending it below 0v. This can happen even with an apparently adequate voltage on the battery pack. Unless you monitor the voltage of individual cells, you will need to stop discharging at a very conservative threshhold, at least 1 V per cell, to reduce the possibility of this sort of failure.

The other way to reduce this sort of failure is to balance the cells with a proper overcharge, 16 hours at 0.1C. This is why we only hear about cell balancing with LiPos, as it happens easily and automatically with lead and nickel chemistries with a simple overcharge, not possible with LiPo!

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Neil provides a complete answer. Be sure to take his cautions seriously, in certain circumstances batteries are dangerous. The cutoff sensor/circuit at low voltages is very important. You really want to avoid "over discharge". In the old days of hermetically sealed NiCds in spacecraft, driving a cell negative, as Neil points out from over discharge, generates oxygen, hydrogen and heat. In other words, it will explode. BTW, you don't have that problem because terrestrial cells are never really "sealed". They have a pressure relief valve.

What I will add, is that you need to look at the full I V characteristic of the solar array (SA) that you are thinking of using. 24 vdc might be voltage where max power is derived from the SA, but in a lightly loaded case the voltage will be quite a bit higher so ensure that your charge regulator can handle that voltage. If at all possible, i.e. if you design your own battery charger, include temperature sensing of the cells, and adjust your final voltage according to the spec for NiMh chemistry.

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