# Current limiting battery charger

Discussion in 'The Projects Forum' started by brywisco, Sep 9, 2008.

1. ### brywisco Thread Starter New Member

Sep 9, 2008
2
0
Hi,

I am a mechanical guy attempting to build a charger for a Lithium Iron Phosphate battery pack. I am using a switching power supply to charge the pack. The PS is 24V, 300W and I have adjusted the output voltage to give me a constant 28.8V. When I connect the battery, the PS delivers 16+ amps!! Once the pack reaches 28.8V, the PS tapers the current off.

I would love to find an easy way to limit the charge current to the battery to around 10-12 A. Am I correct to assume just dropping a resistor in series with the battery would be a huge heat maker?

I am also searching for a way to trip an indicator light (LED?) when the charge current drops to around 0.5A or so.

Please be gentle with me keeping in mind my mechanical training and lack of electrical expertise. Thanks!!!

2. ### iONic AAC Fanatic!

Nov 16, 2007
1,650
95
A little more information would be helpful with respect to the batteries you want to charge. How many batteries are in the pack? What is the nominal voltage of each battery? What is the current capacity of each battery? You may have to refer to the manufacturer for information on charging recommendations. It's hard to know what the ideal current you should be charging with without this information. 10 - 12A seem high to me.

Info to check out: http://www.batteryuniversity.com/

3. ### brywisco Thread Starter New Member

Sep 9, 2008
2
0
I am charging LiFePO4 cells in a 8s3p arrangement.
The cells are 3.2V nominal and charge to 3.6V
This gives me 8cells x 3.6V = 28.8V charging voltage which I set on the output of my switching PS.
The mfgr of these cells recommends a max charge of 3.2A so since I am running a 3P pack, I can do 3.2A x 3 = 9.8A.

My problem with the existing PS is that when I hook up a pack, it delivers 16A at 28.8V. I would love to find a simple way to clip this to around 10A.

4. ### iONic AAC Fanatic!

Nov 16, 2007
1,650
95
Don't quote me, but I think that you may need a bit higher voltage than 28.8 prior to regulating the current down to 10A. I am also having difficulty finding a simple linear regulator that can give you the 10A you are looking for.

Maybe some of the the more experienced members will pick up on this thread and give you a helping hand. I am curious to see what they will find.

May 16, 2005
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6. ### iONic AAC Fanatic!

Nov 16, 2007
1,650
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I have no doubt that "brywisco" will be able to get the 10A from the LM350/LM307 circuit, but what is the relationship between the input voltage and output voltage? Does the LM307 actually boost the voltage so that he can maintain the necessary charge voltage? Looks like a nice solution if this is the case.

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Last edited: Sep 13, 2008
7. ### thingmaker3 Retired Moderator

May 16, 2005
5,073
8
No. But his i/p is 28.8V, so he'll be able to get more than 27V across his battery of cells.

The challenge will be to reconfigure the circuit as a current limiter.

8. ### mack tyner New Member

May 1, 2014
1
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I have several electric bikes with homebrew chargers. I use 12 volt auto headlight bulbs in series with the charger to drop the voltage to the battery pack a little bit, and this also gives the added advantage of a glowing filament that tells me how fast the charging is occuring. The brighter the light, the higher the current. As the pack charges up, the currrent drops, and the light slowly dims and then mostly goes dark. You can experiment with different amperage bulbs. The hot filament also has a higher resistance than a cool filament, so that has an additional benefit of reducing the current more on initial charging, which is usually the problem time. When the pack is depleted, the charging current is higher and heats up both the charger and the battery more than I like sometimes.

9. ### crutschow Expert

Mar 14, 2008
22,862
6,770
To get the desired current you could experiment with putting several auto headlight lamps in parallel. If you can find them, 6V types, such as this, would likely work better and require fewer bulbs for a given current.

10. ### Johann AAC Fanatic!

Nov 27, 2006
190
30
8 X 3.6V = 28.8V is needed as max voltage to fully charge the battery pack and the power supply gives 28.8V. The LM350 needs about 3V between Vin and Vout in order to regulate, thus more than 28.8V will be required as an input to the 10A regulator. (Approx. 33V).

11. ### Alec_t Expert

Sep 17, 2013
10,117
2,468
Have you guys noticed the age of this thread?

12. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
This thread was asleep for 5-1/2 years, and Mack Tyner woke it from the dead.
And it was sleeping SO peacefully!