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A novice question on batteries


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Hello everyone,

I've recently taken up building circuits as a hobby and this is my first post on this forum so apologies if I've placed this question in the wrong category. Anyway here goes:

I'm trying to build a relatively simple solar battery charger. My questions are these:

1) I realize that in order to charge a battery you must meet certain voltage requirements. It is my understanding that if I want to charge say a 9V battery I need a little more than 9 V to do it, 12 or so say. I figure this is because if the potential was higher on the battery than it was from the charger, charge would actually be pushed out of the battery rather than into, the potential being highest there. Is this the correct way to think of it?

2) if it is true that a slightly higher voltage is needed to charge a lower voltage battery, is there a limit on the voltage of the charger that will work? For instance, say instead of charging my 9V with a 12V supply I used a 40 or 50 or even 3000V supply, what is the practical limit of voltage from a supply to charge a lower voltage battery?

3) Assuming you do have the correct voltage to charge the battery, is there any requirement on the amperage? E.g. If you used the proper voltage with only a minuscule current, would the battery charge at all (even if very very slowly) or is there some minimal threshold current needed to charge a given battery as well?

Thank you all for any and every response! 

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A battery is charged with current, not voltage. But of course the voltage must be high enough.
If you don't limit the charging current then the battery might melt or blow up.

Nobody makes a deadly Ni-Cad 9V battery anymore.
A 9V Ni-MH rechargeable battery is made as a six cells 7.2V/150mAh one that is fully charged at about 8.7V or as a seven cells 175mAh one that is fully charged at about 10.15V.

A little 9V Ni-MH battery is not supposed to be quickly charged by a high current. Energizer and Duracell say to charge them at 15mA to 17.5mA for 14 hours if they were dead then to disconnect the charger because a trickle charge is not good. You can charge them continuously with the very low current of only 4mA but it might take weeks for a full charge.

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