# How much power can a lead acid battery dissipate from a non-automatic charger?

Discussion in 'General Electronics Chat' started by rudyauction8, Aug 12, 2015.

1. ### rudyauction8 Thread Starter Member

Jan 27, 2012
252
2
I'm wondering how much power my 36v 7ah and 60v 14ah battery packs can take after being fully charged if I don't shut the charger off without being damaged. I'm leaning towards 500ma on the 36 pack and 1a on the 60v pack. I'm designing a trickle charger and would prefer to not have to build a controller. I'm fine with slow charge (20 hours from dead to full). I spent a half hour on google and couldn't find any info. Note that both packs are exposed to open air outside (or in garage if it's raining), so heat or gas buildup shouldn't be a problem.

2. ### rudyauction8 Thread Starter Member

Jan 27, 2012
252
2
Each pack will have its own trickle charger. The packs are on my ebike and escooter (strapped onto the outside of the frames) and I want a small charger that can be attached to the bike/scooter to be used when I don't have my speed chargers around or just don't need them because I won't be riding for a while.

3. ### k7elp60 Senior Member

Nov 4, 2008
478
69
Lead acid batteries can be charged at a slow rate and the charge rate depends on the amp hour capacity of the battery. If the charge voltage is between 2.25 and 2.3 volts per cell you can leave the charger on for ever. The minimum and maximum current here again depends upon the capacity of the batteries. If the charge voltage per cell is between 2.4 and 2.45 volts per cell then the maximum current is here again determined by the capacity of the battery and the charger must be removed from the battery when the charge current drops to .01C

Last edited: Aug 12, 2015
4. ### nsaspook AAC Fanatic!

Aug 27, 2009
3,006
2,349
For lead-acid it's really a matter of what voltage per cell to maintain full charge. Once you have complete the full cycle bulk and absorption stages the float voltage is selected to match self-discharge if the battery is not being used. I wouldn't recommend a unregulated 'trickle' charger to fully recharge a battery, only to maintain it (with proper voltage regulation) as the charging and float voltages are different.

https://en.wikipedia.org/wiki/Float_voltage

Last edited: Aug 12, 2015
5. ### rudyauction8 Thread Starter Member

Jan 27, 2012
252
2
OK my plan was to build a current limited charger that runs all the time. I'm thinking of building a circuit for each 12v battery that will activate a small load when the battery reaches a certain voltage (13.5v I think is about right). This will also make sure that the batteries all take the same amount of charge preventing any balancing issues, and the trickle charger won't need to be regulated as long as the small load can dissipate the power provided by the charger. Or I have to think about getting out my microcontroller equipment again. For a load I am thinking a car tail light bulb, and a comparator, 5v zener, voltage divider and power transistor (or mosfet) to switch it.

6. ### rudyauction8 Thread Starter Member

Jan 27, 2012
252
2
I should mention that I don't mind a bit of damage to the batteries, I buy them used at the scrap yard for \$0.35/lb, beat them up for a month or so by fast charging at 2c and over discharging them daily, and exchange (with a bit of cash thrown in) for new ones. I just want to avoid blowing them up on the first couple charges. In the end, the used batteries are far cheaper to use then even the cheapest new ones, and I don't like having to worry about damaging a new expensive pack. Plus I have a history of destroying perfectly good batteries by forgetting to take them off the charger.

7. ### nsaspook AAC Fanatic!

Aug 27, 2009
3,006
2,349
If they are already trashed then it won't matter much. I would just make/get an intelligent charger that can't kill perfectly good batteries.

8. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
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Why not just build a voltage-regulated, intrinsically current-limited charger that is adjusted to 2.28V per cell (at 25deg C)? That is a compromise charging voltage which will recharge a lead-acid battery (but not as fast as it is possible to recharge it using a more complex algorithm), without causing serious depletion of the electrolyte if the charger is left on beyond the point where the battery reaches full charge...