Amp-controlled battery charger

Bobb Fwed

I am trying to figure out a way to charge a battery at a certain amperage. It doesn't have to be perfect, and I don't want it to be digital. I'm looking for a better replacement than just a 10/12-ohm resistor between incoming power and battery terminal. I'm thinking some type of voltage divider between incoming power and battery terminal hooked to a transistor.

The idea is, with a resistor, as the voltage of the battery increases, the charging amperage decreases. I want the setup to at least be a little less linear, and try to keep the amperage up (as best we can) as the battery charges (of course until it gets close to incoming voltage).

Any one have any ideas, or existing solutions?

Post Edited (Bobb Fwed) : 6/19/2008 8:45:49 PM GMT

Bobb Fwed

here is the general idea (attached)

bV = battery voltage

Beau Schwabe

In order to maintain a constant current charge on a battery, you need to have a variable power supply.· This is usually accomplished using a fixed voltage supply, and then using PWM (Pulse Width Modulation) in a way that chops the DC voltage to create an "average" voltage·which can be applied to your application.

By monitoring the "voltage" across your resistor, you can determine the amount of current your battery is being charged at. Now as you have indicated, as the battery charges, the voltage observed across the resistor will decrease as well as the charging current, but if you increase the voltage supply as the battery charges, then the current will be constant.

You also should be very aware of the battery chemistry in which you are charging, so that you don't exceed the recommended voltage per cell.· Doing so will cause out gassing, and eventually lead to a useless battery.· Not to mention explosive gases that will be released.


To properly do this, you need to figure the Maximum Charge current that you want, as well as the Maximum Voltage you will be charging at.· Based on that, you can select an appropriate charger power supply that will do the job.· Next finding a transistor bipolar or mosfet (<-preferred) that will be able to handle your current demands.· Usually the rule of thumb is to at least double your current requirement when selecting a transistor.· Lastly, make sure that your resistor can properly dissipate the heat at whatever current you have determined.


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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Phil Pilgrim (PhiPi)

Here's a circuit that should work. The charge current I_C is constant, assuming Vin is regulated and Vin > I_C · R1 + V_bat(max). Neglecting the base current,

····I_C = {Vin – 0.6 – (Vin · R3) / (R2 + R3)} / R1

Since the transistor is operating in its linear region, it may need a heatsink.

-Phil

Phil Pilgrim (PhiPi)

Here's an even simpler circuit that uses an LM317 voltage regulator. The charge current will equal 1.25 / R. Vin doesn't need to be regulated, but must be greater than V_BAT(max) + 3.2V (room temperature, 500mA charge current).

Note: In both circuits, under the specified conditions, the charge current does not diminish once the battery is fully-charged. This may not be desireable!

-Phil

Post Edited (Phil Pilgrim (PhiPi)) : 6/19/2008 9:49:35 PM GMT

Bobb Fwed

ok, thanks, that gives me some good ideas to work with

Mike Green

If you're charging NiMH or NiCd batteries, these are made to be charged indefinitely at 1/10C where C is their stated capacity. A 2500mAh battery can be charged at 250mAh without damage. Lead-Acid batteries are different. You should be able to find charger circuits with a websearch. Also look in the Wikipedia for information on the different types of batteries. They should have articles on Lead-Acid and Lead Gelcell batteries and their charging characteristics.

Bobb Fwed

Yeah, I was planning on charging a lead-acid for a long term usage on battery power. It can peek charge at over 5A, but recommends at or below 1A for best longevity.