Hi Folks,

Attached is a circuit I am currently using. I wanted to have an 'in circuit' battery charger. CONN_3 is attached to the PB137 which is a very handy battery charger. CONN_9 goes to the battery. You can see that when a charger is placed into CONN_10 (can be an AC or DC charger), I am using a 19V computer laptop supply, the battery will charge, even in circuit the battery will get some charge when the plug pack is used. I have heatsinked the PB137 with the 78L05 (I need +12V and +5V) and all seems to work well.

The aim is that the unit can be used on battery and charged while in use if the battery goes flat rather than take it away to be charged.

I could not find anything like this on the web so using the specs from the PB137 I think all is good. My question is is it OK to do what I have done? Any feedback is appreciated.

Thanks

 

Hi Folks,

The aim is that the unit can be used on battery and charged while in use if the battery goes flat rather than take it away to be charged.

Thanks

Since there is no monitoring the Battery Current, in all likely hood, the battery could be sinking or sourcing current all the time, and you may end up with an overcharged or discharged battery.

You need to Monitor the battery Charge / Discharge current.

Ramesh

 

looks ok to me, it should give out upto 2Amps constant voltage of 13.7V, so it alters the current at each stage of the charge cycle, you need to monitor the current drop to sense when full charge is near.

 

Thanks for the quick replies.

I am putting a battery charge monitoring LED across the battery using a picaxe and a tri colored LED which will give me about 8 shades for the different charge states. I could then (possibly) use the picaxe to flash the LED when it goes into to much charge? I could also use the picaxe to drive a relay to disconnect the power to the battery when it goes outside of a window determined by the picaxe?? So after D2 I could put a relay and the picaxe could disconnect here?

I am a bit worried if the charger is left on overnight it may as you say, overcharge the battery.

 

Ok here is what I mean. This is the whole circuit. The battery is a SLA 12V 7AH gel cell. The picaxe at the top is driving the tri-colour LED so I would use the next size up to get a few extra outputs. Hopefully I am on the right track with a 'safe' charging circuit. I have not so far been able to find a good easy in circuit charger so hopefully others could use this design.

Thanks for the feedback so far.

 

Your not monitoring the current, also when the relay clicks in it will remove the 12v supply to the 7805??

 

Only when I have the plug pack in, but that would mean that the battery has reached its 13 volt charge capacity and it disconnects the charger (when in). So with no charger in the circuit still runs from the battery. I will only have a too high charge cutoff.

But like you say, it still does not monitor the current draw of the battery, I will keep searching the wonderful web.

 

Looking more at the PB137 I am using it looks like it takes care of all this! It has internal current limiting, thermal shut-down and safe area protection.