question

ahib avatar image
ahib asked

MPPT SmartSolar charging voltages for a NiMH battery pack

Hello,

Can anyone give me suitable voltages to set for the various custom settings, bulk, float etc (equalization too?) in a MPPT Smart Solar charger?

My NiMH battery pack has a nominal voltage of 12V. It's being fed by a 50W Sun Power panel. There will always be a load (a 10W battery charger), so I just want the 2Ah battery pack to act as a 'stabilising reservoir' so the load gets a nice consistent flow of power.

Thanks.

MPPT Controllerssolar
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3 Answers
nickdb avatar image
nickdb answered ·

You really need to get the spec sheet for your battery and use the manufacturers guidelines.

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The battery pack is just made of 1.2V eneloops. So, the healthy range of charge should be between 10V and 14V for 10x cells. On this basis, can settings for the MPPT controller be suggested?
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seb71 avatar image
seb71 answered ·

NiMH batteries are not suitable to be used in a PV solar system.


If you just want to use energy from the Sun to charge your Eneloops, better option would be to use a 12V lead-acid or LiFePO4 battery for the PV solar system and a dedicated NiMH charger which can be powered from 12V DC (one which has a car adapter).

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Thanks Seb. What's the reason they are unsuitable, if the 12V pack can be charged with a bulk and float charge in the same way as other chemistries?


The long answer (and reason) behind this is that the NiMH cells are to be allowed to drop below 0degC, and then the output/load is to be used to charge lithium ions (in a warmer environment). I've done freezer tests with both the eneloops and a small SLA battery, and the eneloops charged much, much better in the cold.

So, I understand that it's unusual, and there's no preset in the Victron battery menu for charging NiMHs, but I'm not sure I underline the red line that makes them impossible to use with a solar charge controller.

Thanks

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In addition to what kvegermany said, NiMH require a different (way different) charging algorithm. The NiMH charger monitors the voltage and the temperature of the cells during charging. Based on these, it has to terminate the charging at a certain point (and continue with a trickle charging for a certain duration - charging with very low current, like 0.05C).


Overcharging the NiMH destroys them.

Also, if you charge them incorrectly, they can lose capacity quickly (due to the so-called "memory effect").

When charging is complete, NiMH cells should be removed from the charger.


All these make NiMH not suitable for solar systems.

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Thanks Seb. Whilst not at all questioning what you've just said:

For years and years people used to charge NiMHs using dumb timer chargers, which I don't even think had over-voltage cut off.

If it's possible to dumb charge them up to, say, 1.4V per eneloop so 14V for a 10x pack, wouldn't it be possible to give the Victron Smart Solar a similar instruction, so they're not overcharged?

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kevgermany avatar image
kevgermany answered ·

@ahib NiMh requires a different method of full detection. The other cell types work off voltage. NiMh nerds the charger to detect s small drop in current once battery full.

You can install a battery heater if needed. Lead acid works ok below freezing, but at reduced capacity.

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