Solar charging NI-MH

[theRealFury]

Hello All,

I have been using solar for a couple of years now but only been using SLA as storage batteries but i have a requirement for a project of a small solar setup that can handle short duration high current demand, perhaps only 30-40 minutes. Now i know lead can handle it but to size the setup for lead would mean i need a lot of weight and also need to take peukert effect into consideration. I have quite a few spares from different projects and builds so i was hoping to do this with minimal investmant as funds are tight at minute due to my recent ebike build.

My question is i have some NI-MH cells suitable for high drain here that didnt get used in another project and also a solar charge controller with PWM that i can adjust the termination voltage on to a degree to suite other chemistries but i have no idea if NI-MH cells are happy with being solar charged with PWM. Tried searching for my answer but didnt find anything tangiable. Has anyone tried this or does anyone know if NIMH cells can take PWM style solar charging?

 

[amberwolf]

As long as you research NiMH's requirements of temperature and voltage change at end of charge, and build / modify your chargers to suit that, it'd probably work.

If your chargers are simply constant-voltage output, and dont' have a delta-T or delta-V detection for end of charge, you'll end up with a fire or at least damaged batteries, since current flow will INCREASE after end of charge, and be dissipated as heat in the cells (becuase their voltage will drop).

But remember that NiMH has a high self-discharge, for the typical cells you'll find out there. Dunno what the ones you already have are like.

NiMH (and most everything else) is charged via PWM chargers, that then are filtered at the final output of the charger to a regular DC to create the actual charging voltage. If the chargers you have do not have the filtering stage, they'll still work as long as they do not exceed the limits of the NiMH you have for voltage and current. Just won't charge as fast. But if they are intended to be damped by the battery itself, with huge voltage peaks meant to be averaged by battery load to typical charging voltage, I expect that might not be good for the NiMH (it probably isnt' good for lead-acid, either).


Lead-acid is commonly used because it's cheap, most of the time solar stuff is stationary so weight simply doesn't matter, and most of the existing solar stuff is already designed around the lead-acid charging schemes and voltage levels.

 

[theRealFury]

Hey Amberwolf and thanks for the reply.

The NIMH i have is the standard stuff (not low self discharge) and looses about 10% capacity within 24 hours of charging then levels out to loose about 30% capacity over a month. As this will be a small solar system (about 1KW max of storage) that means i would loose about 100watts per day as lost capacity if it was connected to solar each day. And given that these batteries are about 70% efficient in charging im looking at about a 60% efficiency overall. When looking at lead and taking into account its efficiency and for the C rating i would be drawing, taking Peukert into account i think i would probably loose more AH with lead than with NI-MH. The reason i need it to be small is that the power needs to be portable like a small generator so ideally would be in a box that i can carry and then plug into charge when finished.

Now i would just buy a small generator but in the spirit of the environment and in the spirit of repurposing some old bits lying around in my apartment i would like to try and accomplish this small power requirement with solar.

The solar charge controller regulates the current with PWM at the top of charge (basically sending blips of full current for 1nth of a second to reduce overall charge current). So from 13.6v it starts to ramp down the current to slow charging down and by 14.0v it supplies only enough current to float the voltage. My question is would this be suitable for charging NI-MH with a dirty (read spikey) current flow into the battery to maintain the voltage. I could set the SCC to a voltage below the termination voltage of NI-MH so that it never gets to that peak and therefore drop off in power. My hope (not ever using NI-MH large scale like this before) is to be able to float NI-MH at about 80-90% SOC as i hear they can take currents of <0.1C as a constant trickle charge.

 

[amberwolf]

As long as charger stops below final termination voltage of the NiMH, it should be ok, but you should still make something that reads teh thermistor in the pack and shuts off charge based on that, too. Fairly easy, if you first charge the NiMH to full with a regular charger, measure the resistance of the thermistor at that stage, and then make a simple voltage divider that when the resistance is at that value changes the state of a transistor holding a relay in latched state that connects the charger to the NiMH.

Make sure that the circuit can't turn the relay back on if the resistance drops below that value, or it could still become problematic, by cycling the pack hotter and hotter for longer and longer periods at it's core. if you do allow it to turn back on after an overheat, you'll need some sort of hysteresis on that so it can't turn on until enough time has passed for cooling the pack under worst case conditions (or another circuit that reads the thermistor and determines it's passed below a different much lower threshold).

 

[theRealFury]

Right ok, that sounds promising then... ill look into a temperature cutoff for the pack. Im not sure that temperature would be a problem on the pack though as the hghest charging rates the pack should se will be ~ 0.2C (200w of solar pannels and 1KW of storage) so i will have to see if there is much raise above ambient at that charge rate. According to the spec sheet for these cells they can take an infinate duration <0.1c current.

Thanks anyway, its put my mind at rest over using the solar charger on the pack.