I've got a Vsrc of 14.97 crossing a LM317 current limiter ( 1.25V / 1.25R = 1A), and at the output of the LM317 I have 14.4V/1A. So far so good.

Next I use a BC239 NPN transistor to switch a P-channel IRF9630 MOSFET with a 4K7 pull-up. Source is wired to 14.4V positive terminal, and I plan to put a charging load on Drain.

My battery is a 12V/9Ah, so charging would be done at ~C/10, and elsewhere I use a zener diode to read the battery's voltage, and cut-off charging at 13.6V, also cut-off loads at 10.8V (that's done with a relay, it's not relevant to this question).

My question is probably simple to answer: What gotchas should I be aware of when charging a >100Wh battery?

  • \$\begingroup\$ What should the purpose of the 1-2A diode be? What is the type of your battery? Lead Acid, LiIon, ...? \$\endgroup\$ – auoa Feb 5 '17 at 13:12
  • \$\begingroup\$ Lead-acid. To prevent flowback into the charger circuit. \$\endgroup\$ – user400344 Feb 5 '17 at 13:13
  • \$\begingroup\$ E.g. if my Vsrc (currently a laptop charger, later it will be a solar panel with blocking diode) goes low. I don't want gremlins in my circuitry. \$\endgroup\$ – user400344 Feb 5 '17 at 13:22
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    \$\begingroup\$ Please draw a schematic. \$\endgroup\$ – winny Feb 5 '17 at 13:49
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    \$\begingroup\$ If you haven't browsed Battery University web site already, it's a mother lode of information on battery issues: Charging Lead Acid. \$\endgroup\$ – Nick Alexeev Feb 6 '17 at 4:35

Charging a Lead Acid battery is quite easy. You should first apply a constant current (CC) of max. 1C (in your case 9A) until the voltage reaches 14.4V. Hold this voltage for at least one hour, at most 10 house to balance the cells inside the battery. At the end of the constant (CV) period the current should be below 1/20 of C (in your case 450mA). Do not apply voltage above 14.4V, this will reduce the capacity of the battery over time because of water loss.

  • \$\begingroup\$ That's a good answer. And what if I maintain charge at 14.4V/1A for the duration? \$\endgroup\$ – user400344 Feb 5 '17 at 13:25
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    \$\begingroup\$ So your gonna charge with 1A in the constant current case? Just charge as long as the battery needs to reach the 14.4V. \$\endgroup\$ – auoa Feb 5 '17 at 13:27
  • \$\begingroup\$ Okay. And no need for diode blocking from the positive terminal of battery to MOSFET drain? I plan on making permanent connections to the battery, so there will never be sparks flying when I attach clamps. \$\endgroup\$ – user400344 Feb 5 '17 at 13:31
  • \$\begingroup\$ I'll sense voltage at intervals of 600 seconds, having cut off charge 30 seconds before sensing. Am I alright here? \$\endgroup\$ – user400344 Feb 5 '17 at 13:35
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    \$\begingroup\$ Your cap is far to small for this old linear regular, try at least 4.7uF at input and output of the LM317 \$\endgroup\$ – auoa Feb 5 '17 at 16:58

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