# Looking for help to build a battery charger

Discussion in 'Power Electronics' started by Devil Master, Jul 30, 2011.

1. ### Devil Master

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0
Jul 30, 2011
Hi, I'm new here, so please don't bite the n00b.
I have a DC motor with a nominal voltage of 48 V, which draws a current of 2.55 A, which I want to try to run with batteries. I've read that NiMH batteries are easy to make a charger for, and I found a store that sells 1.2 V NiMH batteries with a nominal capacity of 4000 mAh.
The "running the motor" part looks pretty straightforward (connect 40 of those batteries in series and you get a voltage of 48 V), but after a search on Google I realized that I know next to nothing on how to make a charger. Of course, I want a single charger, to charge all the batteries while they are connected in series (the prospect of having 40 chargers, each for a single battery, would be very impractical ). I know that I need to step down the voltage from 220 V (I live in Europe) to 48 V and use a full-wave bridge rectifier and a capacitor to obtain an approximately DC signal, but then... what?
I know that the theory says that the current needs to be constant, while the voltage must be increased gradually from a little more than the actual charge of the battery to the nominal voltage. But how do I put this in practice? Is there a "generic" battery charger circuit scheme, where the values of resistors and capacitors can be obtained with simple formulas from the values of the nominal voltage and the desired current?

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
NiMH chargers are actually non-trivial.

Why not look at 4 x 12V lead acid batteries.

At worst, you could then simply attach 4 chargers up to this. Depending on their price, it may be the most efficient way to go.

3. ### Devil Master

3
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Jul 30, 2011
Uhm. That's an option.
If that is the worst case, it means that in a non-worst case, it's possible to build a battery charger for them.
I found this page for the construction of a charger for lead acid batteries. Is it a good guide?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Simple lead acid chargers can cause damage to the batteries. It all depends on the situation. If you're using flooded cells, you can tend to get away with more overcharging than you can with sealed cells.

You generally require some amount of overcharging to "equalize" the cells.

If the temperature is constant, you may be able to get away with float charging them with a constant voltage. Otherwise you need (at the least) something which varies the charging voltage with temperature.

Better still, you have a charger which has several phases of operation, depending on the charge state of the battery.

5. ### Devil Master

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Jul 30, 2011
By accidentally overcharging them?
These are the batteries I would use. They are sealed.
"With a 6-cell (nominal 12V) lead-acid battery the correct float voltage drops by about 0.15 V for a 5°C rise in ambient temperature."
I would charge them in my house (heated in the winter and air conditioned in the summer), so I'm assuming that the temperature would remain pretty much constant. But how would the nominal variation differ from the values taken from Wikipedia, if I charged 4 batteries in series (48 V) instead of one?

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Overcharging flooded lead acid batteries is no big deal because they just liberate H2 and O2 and you top them up with distilled water. Sealed batteries are a problem in this regard because you can't top them up (and the electrolyte is a gel)

Get the specs on the batteries for float charging them. It will be a particular voltage (at a particular temperature and possibly max current).

4 batteries in series will have 4 times the voltage change per degree C.

Charging batteries indoors is considered a bad idea because H2 and O2 in combination are highly explosive.

Note that your charger will have to be able to supply at least 2.55A if the motor runs continuously. If not, it can supply less but remember you have to put in about twice the amount of energy you take out to recharge the battery.