# Charging batteries - what should I know

So I want to learn about batteries and the process of charging them.

As a project, I'm going to create a solar powered charger to charge a 9v NiMh battery with 175mah capacity.

I have a 15v solar panel which produces 0.32A of current. I will use this to charge my battery.

So here are my questions:

• I read somewhere on the net that you should not charge a battery with a higher voltage than its capacity. So if I was to just connect the panel to the 9v battery, while it's generating 15v - boooooooom? I should use a 9v voltage regulator, right?

• I also saw something on the net about only charging batteries with 10% of their current. Is this true? Should I be resisting my current to 17.5mah (for a 175mah battery)? If so, why?

• 15v is the peak voltage of the solar panel. It won't always be at this rate. Infact, it may even be lower than 9v. Does this mean that if the current battery charge produces more power than the solar panels, the charge will actually travel in the wrong direction and damage my panels? Should I be using a diode and/or something else to prevent this?

• How do I work out the amount of hours required to fully charge the battery?

• If something was to go wrong with this project, could I blow up my whole house and kill myself and invalidate my home insurance and make my family homeless? No seriously, is it safe?

• Is there anything else I should know (saftey, good practices, etc etc)?

• Not blow up the whole house... but no harm (heh) trying ;-) – Anindo Ghosh Aug 16 '13 at 18:09

Charging NIMH (and most modern battery chemistries) isn't done at constant voltage, but at constant current.

You'll need some fairly involved electronics to provide a constant current source and monitor the battery state.

Battery University has a pretty good description of the charging cycle of NIMH batteries.

The initial charge is at 1C (in your case, 175 mA). There are 3 different stop-checks (whichever is reached first):

1. Temperature rise rate of the cell being charged.
2. Voltage across the cell.
3. NDV (negative delta voltage). Basically check for when the voltage rise of the battery stops rising at a certain rate.

At this point, the charger will charge the battery at significantly less current, repeating similar 3 checks and iteratively stepping down the current charge rate.

The linked page suggests that slow-charging a NIMH battery is difficult/impossible because it's very difficult to get reasonable readings for many of the time-dependent stop checks. You're basically relying on a time charge, which could excessively charge the NIMH cell and damage the cell.

The primary safety concern is excessively heating up the battery. You might risk rupturing the battery, though I don't think NIMH are susceptible to catching fire as some other cell chemistries are. Most likely the battery casing is designed to rupture in a safe manner, but no guarantees.

It's unlikely you'll do anything which would be considered a significant safety concern, just use your common sense. There aren't any high voltages involved, and the power outputs are relatively low.