Charge Current Controller for SLA/Gel in a vehicle

[OregonJim]

I need to build a circuit to allow me to limit charge current to a SLA or Gel battery connected directly to a vehicle DC system when the car is running. I want to be able to limit the current to no more than about 8 amperes so that if I run my SLA/Gel down, the alternator won't try to charge it back up at 30 amps (or more).

Obviously I want to keep the SLA/Gel charged up but not destroy it with too much current. The battery I have now is rated at 7.5A (8-hour rate), but would like to be able to change that based on other batteries.

Anyone know of a nifty design that is relatively immune to temperature changes, and hopefully not a source of RF noise? This will be used to run a Ham rig, both when the vehicle is moving and also when it is stationary.

Thanks a lot!

Jim in Oregon

 

[crutschow]

You might try a hot-swap controller. They are designed to connect two circuits with minimum voltage drop while limiting the current. You need one that can tolerate at least 20V.

 

[MikeMl]

Since the car's electrical system is regulated to 14.2 to 14.4V, and it takes 14.7V to charge the SLA, a switcher is called for.

Perhaps you could put some big Silicon diodes between the output of the alternator and the car's lead-acid battery (requires a remote sense input to the voltage regulator). This would give you more voltage out of the alternator for charging the SLA, and sufficient voltage headroom for an electronic current regulator.

Another idea: instead of a single 12V SLA, use two 6V ones, and charge them in parallel. While the car is running, you dont need the SLA(s) anyway...

 

[crutschow]

Since the car's electrical system is regulated to 14.2 to 14.4V, and it takes 14.7V to charge the SLA, a switcher is called for.

Why do you say it takes more voltage to charge an SLA than a standard lead-acid battery? They both have the same basic battery chemistry and the same voltage.

 

[MikeMl]

Why do you say it takes more voltage to charge an SLA than a standard lead-acid battery? They both have the same basic battery chemistry and the same voltage.

AFIK, it is because they are not quite the same chemistry. SLAs use more Calcium in the plates (or is it the other way around). If you actually look up the data sheets from SLA manufactures, you will see recommended cyclic (fast) charging voltages of 14.5 to 14.8V. If the charging voltage of an SLA is constrained to the recommended float-voltage of 13.5 to 13.8V, it takes forever to fully charge them...

 

[crutschow]

OK, manufacturers might recommend slightly higher maximum charge voltage, but I would think they will still charge reasonably rapidly at 14.2 to 14.4V, at least fast enough to not have to go the complication of a switcher or other circuitry to boost the voltage a few tenths of a volt.

 

[k7elp60]

I have built a number of SLA chargers for batteries. Most of them were set at the float charge voltage,(2.25-2.30) per cell.
Most SLA don't require current limiting at this voltage. A lot of my charges had a current sense that turned on an LED when the charge current dropped to the capacity/100. For a 7.5AH battery this would be 75 milliamps. I have seen the current drop as low a 10Ma during charging. This method of charging is good as you don't have to disconnect the charger and the battery will not overcharge.

I have also built a number of chargers that charge the SLA from the cigarette lighter socket. The batteries ranged from 18AH to 70AH. One thing to consider is never connect a SLA directly to the cigarette ligher socket. The battery may well charge while the engine is running. If you forget to remove the connection and try to restart the car the SLA battery will supply enough current to burn of the wires to the cigarette lighter socket.
One method I have used on a number of times is to forward voltage match some 3A schottky diodes, put them in parallel and inseries with a circuit beaker or fuse to the cigarette lighter socket. This method works if the voltage output from the cigarette lighter plug is close to 14.2V with the charging current load.
Another scheme is to use commercial isolated switching 5V supply to put in series with the cigarette ligher plug and then use this as the supply voltage to the charger.

The best technical info I have found is from Powersonic and is attached.

 

[Mark_R]

I'm using this --> **broken link removed**

Very sophisticated charge profile for the $11.

9-36V input for charging a 12V SLA up to 7AH

 

[k7elp60]

I'm using this --> **broken link removed**

Very sophisticated charge profile for the $11.

9-36V input for charging a 12V SLA up to 7AH

An interesting module. If I read the information correctly if could not be used in a common ground situation. That is when the battery - terminal and the powersupply - need to be common. I seriously doubt it would work in a automobile situation where one wants to use a seperate battery for their radio that has a common ground.

 

[Mark_R]

An interesting module. If I read the information correctly if could not be used in a common ground situation. That is when the battery - terminal and the powersupply - need to be common. I seriously doubt it would work in a automobile situation where one wants to use a seperate battery for their radio that has a common ground.

I don't kow if this would help; from Pg 5 of the data sheet -->
"Other application drawings using a single pole change-over relay or solid state switching (MOSFET) are also available."

In my application, the isolated battery is fine (even desirable) so I have never requested info on other possible configurations.
I spend days trying to "roll my own" charge controller that would provide battery monitoring and intelligent charge control. I never got close to this units functionality for the money.