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Battery Charging Problems

Lord_grezington

May 3, 2013
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Hi there

I am trying to get a battery charging circuit designed, however as I am not an expert on batteries I have a couple of questions.

1. Can I charge the battery via PWM? IE, 12V lead acid requires 14.4V to charge, so can I use a PWM voltage at a 30% duty cycle (from 48Vdc) without damaging the battery?
1b. Will the same theory apply to lithium batteries?

2. Batteries typically have a low resistance, lets say 1 Ohm. so if I then put 14.4V across it, there will be a current draw of 14.4A (which is too high). I then regulate this current draw to say a 5A max so my PWM signal is now @ 10.4% (of my 48V).
2b. Will by battery now stop charging because my voltage is not high enough?

Thanks
Graham
 

Harald Kapp

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Nov 17, 2011
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1) That's no good idea. Even if the battery sees the 48V only for short pulses, it will draw a lot of current duríng that time and will overheat.
1b) Don't even try this with lithium batteries, they can (and will) explode - see the Boeing disaster.

2) Don't rely on "typical values" with batteries. There is a reason that there are quite a few specialized charger circuits (and ICs), each one for dedicated battery chemistries.

Some batteries need constant current charging, others need constant voltage charging with or without current limiting. All batteries need some kind of charge-completion detection to prevent overcharging. Additionally overvoltage and overtemperature detection may be in order (especially for lithium based batteries).
Go to the battery university and learn about different batery chemistries (at least the ones of interest to you) and charging methods.
It will also be worth your time to browse some IC vendors for charger ICs, e.g. Maxim, Linear Technology, Analog Devices and others (Google).
 

Lord_grezington

May 3, 2013
124
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May 3, 2013
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124
Hello Harald

Thank you for your prompt response.

I will smooth the PWM out by using some large capacitors - its costly but if this is what it needs....

I am designing the circuit to have both constant current and constant voltage to try and keep the flexibility with battery options. I will also have the charge completion detection in place.

I have thought about using the dedicated IC's, but they are not very flexible. I might give the battery university a call for some details.

Thanks
Graham
 

BobK

Jan 5, 2010
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If you must charge from 48V and use PWM, use an inductor after the PWM signal and you will have a buck converter.

Bob
 

Lord_grezington

May 3, 2013
124
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May 3, 2013
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Hi Bob, buck converter works well on spice. I will get some bits in and see what I get.

Graham
 
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