Charging Current for a SLA battery

5 replies to this topic
  • DSTerry

Posted March 16, 2006 - 02:21 PM


I was looking at the data sheet on the 12V 1.2AH SLA battery I am purchasing, and it states a maximum charging current of .36A.

If I am running a 10W auxillary headlight bulb, and a tailight pulling around 3 watts, that is a a drain of 13W @ 12v = 1.08A. So if my battery is charging at .36A and I am discharging it at 1.08A, it seems like the battery will eventually drain. So it only makes sense to let the battery charge at a higher current since there will be about 4 to 5 amps available. Don't know what the consequences would be with this particular battery.

The discharge time at .72A is around 30 minutes for this battery. I need to look over everything again. I'm just not sure how those Regulator/Rectifier all in one devices are designed.

Their must be some extra circuitry in their to accomodate for these things.

  • DSTerry

Posted March 16, 2006 - 04:53 PM


I talked with BD, and they told me the battery in their systems would never see more than 1A of current. I wasn't quite sure how to take this. They may have some current limiting circuit in the regulator/rectifier. Though this would not make sense because if you limit your DC system to only 1A, then the battery would absolutely be drained very quickly. Especially with the BD system since they also run their headlight from the DC side. 1A at 12V will only give you 12watts of power.

So the only other thing that makes sense is that they don't current limit the output of the regulator, but assume that since the headlight, taillight, and all the other lighting will be drawing the extra current, which may leave approximately 1A for the battery to charge. Thus the lighting will not discharge the battery, but run directly from the full current supplied from the stator.

If this is the case, what happens if you turn of your headlight and running lights and decide to ride through the desert for an hour or two with no lights. If the stator is putting out 4 to 5A at 12v, will the battery overcharge since typically when you charge an SLA or Nicad battery, you only want about 1/5 to 1/10 charging current compared to the AH of the battery. And most high power fast chargers have a circuit on them to charge for a given amount of time, then turn off.

I may be overthinking this, but I'm an engineer, so thats what I do! I just don't have all that much experience with batteries. The BD system seems to work fine, so maybe I'll just run their regulator/rectifier and put my headlight on the DC side like they do. But I would really like to know what is inside one of those regulator / rectifiers so I can better understand how this whole system works.

  • qadsan

Posted March 16, 2006 - 05:27 PM


...But I would really like to know what is inside one of those regulator / rectifiers so I can better understand how this whole system works.

I believe many of the regulator rectifiers for various off road bikes are simple crude designs such as 4 diodes for rectification and the battery itself is used to reduce ripple in addition to storing energy. The regulation is likely as simple as SCR's that are used as a shunt so when the voltage reaches a certain point (i.e. 14 volts), then the excess current is diverted away from the battery. These types of components can be put into a fairly small package for a very low cost.

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  • DSTerry

Posted March 16, 2006 - 10:33 PM


Thats pretty much what I thought all along, but the guy at Baja Designs kind of confused me a little when he was sure the battery would never see more than 1A. I just don't see that happening with a simple rectifier circuit. A rectifier and regulator will not limit the current, just convert the AC to DC and limit the voltage. So if your stator is putting out 5A, that's what the battery will see assuming their is no other load (lights turned on).

Before I saw these all in one pacakges, I was going to build my own full wave rectifier and add in a regulator, but no need to with these nice packages. I just want to make sure I understood what was going on before I added it to my system.


  • cleonard

Posted March 17, 2006 - 12:40 PM


I wouldn't sweat the charging current too much. To limit the current on the initial charge you just need like 1 ohm or so. Perhaps a long piece of smaller gage wire.

The BD(Baja Designs) regulator is more than just a rectifier. The output voltage of the alternator is proportional to RPM. I measured like 60V open circuit on my lighting coil at high RPMs. The DB regulator is also good for much higher currents than just 1 amp. In my old design system it also ran the headlight. That's 5 amps or so. Plus is has to have some extra for the tail light and to charge the battery at the same time.

The end result is that you will be running the battery out of it's specs. I think the only spec that will be violated it the charging current. With the BD nicad pack they are violating the charging method, charging current, and discharge current. The spec sheets for nicads have big warnings for constant voltage charging. Not just a "we don't recommend it." More like "DANGER!!! Constant voltage charging could result in a fire, explosion, etc." However, I doubt that anyone with a BD kit has had one of those things happen. The excess charging current will reduce the life of the battery a little.

  • DSTerry

Posted March 17, 2006 - 04:20 PM



Thanks for the info on the BD rectifier/regulator. I will most likely purchase this one. I am running the stock stator with the stock 35W bulb. The system should work great with the 35W headlight and 3W tailight on all the time. These will be drawing about 3.2A. I think the stator puts out around 55W so at 12V thats around 4.5A. This leaves a little over 1A for the battery to charge. That is not to far out of spec.

I'll even be able to run my headlight for about 5 min with the engine off if the battery is fully charged.

Thanks again.

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