Cheap Chargers - Trimming Voltage

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DrkAngel   100 GW

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Cheap Chargers - Trimming Voltage

Post by DrkAngel » Apr 03 2013 8:21pm

I found some cheap, reputable brand name Li-ion chargers.
eBike Toolbox Bargains

$24 for a 25.9V 2A Li-ion charger.

$18 for a 36V 2A Li-ion charger.

Sadly, they are fixed voltage, 4.2V per cell, with no trim pots.

Of course I could probably go fishing with a resister till I found a point that I could pot for trimming.

But ...
Also found some cheapo 3A blocking (directional) diodes.
"Good" quality will reduce (trim) voltage .1 - .2V.
These lesser quality will drop voltage .35V.
So ... with a 7s 29.4V supply (4.2V per bank) 1 diode will drop voltage .35V to 29.05V (4.15V per bank).

Yes! They can be run in series!
25.9V (29.4V charge voltage)
7s 25.9V OEM = 29.4V (4.20V)
1 in series = 29.05V (4.15V)
2 in series = 28.70V (4.10V) "Preferred" for certain Li-ions
3 in series = 28.35V (4.05V) Ideal for my recycled Lipo!

36V (42.00V charge voltage)
10s 42V OEM = 42.00V (4.20V)
1 in series = 41.65V (4.165V)
2 in series = 41.30V (4.13V)
3 in series = 40.95V (4.095V) "Preferred" for certain Li-ions
4 in series = 40.60V (4.06V) Ideal for my recycled Lipo!

50 PCS IN5408 3A 1000V Rectifier Diode - $4.27

These should work nicely, up to 3A ... of course you could run in parallel, for 6A, 9A etc.
Update - running diodes in parallel not reliable or effective!
Fortunately, they are available in 1A 3A 6A 10A etc.

Rather than implementing inside the charger, I think I will continue my "modular" bent.
A 2 - 3" segment with deans plugs on each end.
That way I can quick modify a charger for various voltages.

Still have to determine if a heatsink required?
Update -
2A charger x 29.6V = 59.2W
2A x .35V = .7w heat production?
If figured correctly, heat output appears to be negligable.
Even with 2-3 in series, I doubt there will be any problem.
I had intended on surrounding with heatshrink tubing, thinner black electrical tape might be better?
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