NIMH pack charging/improvising

[Lumencraft (Matt)]

Hi everyone,

I need to know a bit more about charging NIMH packs. The science behind it general ruels and mostly how to improvise.

I am familiar with the basic do's and don'ts, of using different chemistry cells, and the voltages. I have done some small improvisions like using jumper wires to charge cells that are too large etc. But what I am wanting to do is go from using individual cells to packs. I realize the best way is just to get a couple of good hobby chargers ( I am working on it) but in the mean time I'd like to learn a bit more, and maybe charge a few packs. :naughty:

My first question is approximately how much higher should the output voltage of the charger I am using be than pack? How high is too high?

If the voltage of the charger (or power supply that I am monitoring Very closely) is not higher than the desired termination voltage of the pack will there be damage to the pack? Example. If I have a pack with 9x nimh cells and the desired termination voltage is 1.4v per cell (hot off the charger) for a total of 12.6 volts, and I hook up a power supply ( I know charging will not terminate on its own) that outputs only 12v 1a then when the pack hits 12+ volts will I get that reverse charge thing going through the cells like if you use a cell that is dead in combination with new ones?

Thanks in advance for your help.

 

[KiwiMark]

I realize the best way is just to get a couple of good hobby chargers ( I am working on it)

Are you aware of this product? I recently got one and am very impressed - I would have bought one of these a while ago if I realised that they were that cheap (and I have 4 chargers that I would not have bought). You do need 12DC to power it - but if you have access to a working PC PSU then you can easily connect the charger to it (yellow = +12V, black = ground) I use a PSU with the green wire shorted to the black to turn it on.

 

[firewire_bf]

The science of charging NiMH cells and packs is really simple. There are a couple of rules:
1. avoid charging currents higher than C (capacity of a single cell)
2. avoid cell temperatures higher than 45 degrees Celsius (113F)

The voltage over a single cell will end up around 1.45-1.5V normally. It depends on the charging current. If your charger can't reach that voltage then the batteries would probably not be fully charged. Again, It depends on the current. In your example you have a pack with 9 cells and a 12V 1A power source. Cell voltage will end up around 12/9 = 1.33V. Which is higher than the nominal 1.2V, but lower than 1.5V. This means that they will be charged to a certain level. If you can monitor the charging current it will be easier to determine the state of charge. Lets say you begin charging a depleted pack. The current will be fairly high<C. As the pack is receiving charge, the voltage will rise, and the current will drop. Now if the current becomes zero, then you know the pack isn't full and will never be fully charged with that charger. Otherwise the current will stay just above zero, trickle charging (around 0.01C), and you will have a good change of fully charging the pack, though it will take several hours to fully charge.

The cells will never reverse charge during a charging process.

/Emil

 

[Lumencraft (Matt)]

Are you aware of this product?

That is similar to what I am looking for but, I need to be able to charge between about 4-15 cells or so.

Lets say you begin charging a depleted pack. The current will be fairly high<C. As the pack is receiving charge, the voltage will rise, and the current will drop. Now if the current becomes zero, then you know the pack isn't full and will never be fully charged with that charger. Otherwise the current will stay just above zero, trickle charging (around 0.01C), and you will have a good change of fully charging the pack, though it will take several hours to fully charge.

The cells will never reverse charge during a charging process.

/Emil

So am I correct in assuming that if I use a 12v 1a power supply rather than an actual charger the current will coninue reguardless of the pack voltage?

Also what is considered too high charging voltage? Example: My screw gun is 14.4v but the charger puts out somthing like 18v a difference of 3.6v. How large of a gap is considered safe? If I were to charge a pack that was only 10.8 volts (with a desired termination voltage of 13ish volts) Will that dammage the pack? How many volts over is ok?

 

[buickid]

Grab one of these. Or just invest in a real RC hobby charger. Its worth it if you're going to be charging cells a lot. They're not cheap after all!

 

[alpg88]

i used to make packs for my projects, but than i got tired of opening packs and removing dead cells, sure getting cells from reputable manufacturer will decrease chances of a dead one in the pack, but still possible, now i invest extra time, and build holders, so i can charge cells separately.

 

[firewire_bf]

The current will flow as long as the open-voltage of the charger is higher than the open-voltage of the battery pack. In your case, vestureofblood, the current will flow from the power supply as long as the voltage of the battery pack is lower than 12V. The voltage of the battery pack will never go above that. At this point, only self discharge process of the battery will draw current from the charger.

The current flow is depending on the open-voltage difference between the battery pack and the power supply, also on the current limit of the power supply.

If your power supply gives a sinusoidal voltage of 12Vrms then the peak voltage would be around 17V. With a reverse protection diode in series with the battery you may fully charge your pack.

A charger needs over-voltage to be able to drive a current through the battery. The batteries and everything else in the circuit have a sum of resistance. The over-voltage divided by that resistance gives the charging current. To high current will kill the battery. So to limit this current you may limit the over-voltage or add some resistors in the circuit.

In your example with the screw gun charger. The over-voltage is 3.6V. I assume nice DC. And the 14.4V battery may have 12 cells with internal resistance about 10mOhm each. Then that pack would have at least 120mOhm of resistance. The charging current would be around 3.6V/0.12Ohm = 30A. This current would kill the pack in no time. Luckily the power source to that charger is limited and it would most likely contain a circuit limiting and monitoring the charging current and the state of charge of the battery.

To charge a 9 cell pack on this 12 cell charger you would have to monitor the battery voltage and temperature. In the end of charging process the pack voltage would probably reach around 1.5x9 = 13.5V. Then the excessive charging current will build up heat in the batteries. If they feel warm, not to hot, the pack is ready and you should break the process. Don't expect the charger to stop charging automatically.

/Emil