Well, I have a 12V 9AH SLA Battery, and I use a 7815 + a diode in series to charge it, (I really wanted to build a charger using the L200 or the LM317 but I couldn't find any of those in my city). So, I've read "Battery University" and I ended up with more questions than before.

First, It states that the open circuit voltage of a fully charged battery is 12.65V however, mine came with an open voltage of 12.93V, (And the delivery took a whole month) was it overcharged or is it its real 100% charged open circuit voltage?

Also, I've read that the battery shouldn't reach more than 14.4V to prevent gassing, however mine even without the diode (I know it is risky without the diode if I unplug the charger) doesn't get close to that voltage, after hours of intentional "overcharge" the highest voltage that I could reach was 13.5V and it didn't go any higher. then after unplugging the charger and waited a couple of hours, I measured its open circuit voltage, which was 13V. (Which seems to be the real 100% charged voltage of this battery, as it measured 12.93V out of the box after who knows the last time that it was charged).

Also, when it is charging I wait until the current drops to 2% of its amp-hour capacity, however if I wait an hour and try to charge it again, the current jumps to 0.7 amps and stays that way for hours until it again settles down to 2%, it seems that it could keep doing that forever.

(By the way battery university states 3%, while "Lee Hart's Battery Charging Basics" states 2%, who is right? xd)

Am I destroying the battery? My meter measures temperature and the battery temp always stayed at ambient temp, even when "overcharging" it.

And finally, what is the float current of the battery? Because I want to float charge it by simply adding a resistor in series to the charger, instead of adding a bunch of diodes in series to reduce the voltage.

Right now I treat it as an old battery (even though it supposedly isn't), I wait until the current stops falling (which happens really close to 2%) and I unplug the battery, however, shouldn't an old battery open voltage be 12.9-13V?

  • 2
    \$\begingroup\$ You overthink things ! 12.65V however, mine came with an open voltage of 12.93V, Well that voltage is also temperature dependent, yours maybe had more charge and charge leaks away over time. Don't worry about that 0.3 V difference, it is insignificant. SLA batteries are not that fragile. If you charge it to a maximum of 13.8 V you will be fine. float current does not exist, you mean float voltage, that is the voltage without loading the battery. You cannot charge from 15 V with some resistors in series because the voltage will rise above 13.8 V and that is a no-go. \$\endgroup\$ May 23, 2016 at 9:47
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    \$\begingroup\$ You can use that 15V to charge your SLA Battery but instead of only one diode, put two in series that way you will get a voltage of around 13.6 - 13.8 V which is absolutely fine and will not overcharge your SLA Battery. With 2 diodes in series, charging will stop at 13.8 V which is what you need. You can even leave the SLA Battery connected all the time, it will not be overcharged. \$\endgroup\$ May 23, 2016 at 9:52
  • \$\begingroup\$ I was going to tell you that without load each diode dropped 0.2V, however I just tested with a 2K ohm load and it dropped down to 13.72V. (I'm stupid xd). Thanks. \$\endgroup\$
    – Phoenix1
    May 23, 2016 at 10:27
  • \$\begingroup\$ OK, I was talking about Silicon diodes like 1N4001, your 0.2 V indicates that it is a Schottky diode, these have a lower voltage drop. Of these you need to put (15 - 13.8) = 1.2 / 0.2 = 6 in series. \$\endgroup\$ May 23, 2016 at 11:22
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    \$\begingroup\$ For faster charging, you may want to use one diode in series with your 7815. Then after some period of time, you can switch to using two diodes in series for float (maybe use a transistor to bypass one of the diodes for a few hours). Check the float voltage with a meter. It may creep up over time. Also, there are a few different types of lead batteries. Is yours a standard flooded battery? The most sensitive types are "gel cells." If you have a "gel cell" then make sure you follow the manufacturer's recommendation for maximum voltages. \$\endgroup\$
    – user57037
    May 23, 2016 at 15:58

1 Answer 1


Can't help with the chip/diode specifics, but here's some battery basics.

Each batt model has different specifications for Absorb voltage, Float voltage and endAmps, the tapering current point defining Full, at which voltage is allowed to drop from Absorb to Float.

I would not buy a battery where those specs are not available, as well as e.g. charts showing avg DoD impact on lifetime cycles, temperature vs discharge curves, etc.

But faced with an unknown, look for that data on similar batteries from more diligent vendors, and approximate is good enough.

To get a decent charge, current rate needs to also be high enough. Cheap lead can't accept more than .2C anyway, so for a 9Ah batt you should use at least an amp, but over 2A would be better for testing.

Your current rate and short charge time likely means you never got anywhere near Full, which shortens lifespan cycles.

So, just as an example:

  • Absorb 14.4V - 14.7V
  • Float Voltage = 13.8V - 14.1V
  • endAmps = .005C
  • Max Absorption Time = 12 Hours or, Current change over 1 hour period of less than 0.05A
  • Temp Compensation = -.018V for every degree °C rise above 25C

So, discharge the battery to 50% SoC. If you don't have a proper discharger or battery monitor SoC gauge, a cheap Ah meter will do.

Set a power supply to 2-3A @14.7 max, measure V and A as SoC climbs during Bulk/CC stage you'll see V climb. When 14.7V is reached, probably less than 2 hours, that's now Absorb/CV stage, maybe 4-6 hours remaining to get all the way to 100% Full, not sure used to working with 300-800Ah banks.

You'll see amps dropping, the stop-charge spec is .005C, so for 9Ah that's 50mA.

But an old / worn / poor quality batt may not get there within the maximum 12 hours, so if you see the current just sitting, no longer dropping by over 50mA per hour, you can use that as your Full point.

Measure the actual SoC with precision if you can when the batt is new, at the 20-hour discharge rate down to 10.5V (0% SoC), will be a bit higher or lower than 9.0Ah.

From now on, if using different current or voltage rates can be used, that Ah capacity benchmark becomes your objective measure of 100% Full.

As the battery ages, the point where that capacity drops, that % aka State of Health, standard EoL is 80%.


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