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danielwritesbac 31st March 2012 06:55 AM

Float charger? Cut off at 13.5v rest until 12.5v before switch on?
I'm trying to replace the old style charger in an RV, with something that doesn't cost $20 per month to run or make a nonstop hum.

So, possibly a float charger that turns off when it isn't needed, and maybe power the charger from an inexpensive SMPS laptop power pack?

It would also be useful to relay switch off the main converter's ac input while the battery is within float range, since both converter and charger should be off while the battery has at least 12.6v~12.5v.


danielwritesbac 30th May 2012 07:27 PM

Well, it does seem that a little attention to detail would be more cost effective than buying batteries every 3 years after they are wasted by poorly done charging.

gmphadte 31st May 2012 10:23 AM

U need a window comparator circuit, which can be made from comparator ICs like LM339/393.

Gajanan Phadte

sofaspud 31st May 2012 11:09 AM

The OP description isn't really a float charger, which ideally is charge rate=self-discharge rate. Otherwise, I agree a window comparator is in order, but if the supply input is to be cut off then an auxiliary power source is required. That might be easier to do with a CMOS watchdog and alkaline cells rather than some sort of monitor-its-own-voltage-source circuit.

PChi 31st May 2012 11:30 AM

I was under the impression that 'Float Charging' was applying the charging voltage continuously which needs to be well regulated and ideally temperature compensated.
To maximise the battery life by preventing under charging it helps to have a boost charge facility as well.

There are some integrated circuits around to do this such as the Texas Instruments bq24450. I guess that there are others. It's possible to roll your own with comparators and someting like a 723 Voltage Regulator but why bother.

Ouroboros 31st May 2012 12:39 PM

I guess this is not a Lead-acid battery? If it was then there would be no point in turning the charger off, just ensure that it has a properly temeperature-compensated float voltage. The temperature sensor needs to be a remotely connected one so that it can be attached to the battery (or batteries) under charge.

A Ni-Cad or Lithium rechargable battery pack will need to be disconnected when the batteries reach full charge, but each of these battery technologies require their own dedicated charger.

gmarsh 31st May 2012 12:52 PM

Something like this?

AC --> AC relay contacts -> 19.5V AC adaptor --> LM317/350 configured as a current regulator --> reverse voltage protection diode --> LM317/350 configured as a voltage regulator ---> 12V battery

12V battery --> comparator circuit w/reference and hysteresis --> FET --> AC relay coil

sofaspud 31st May 2012 04:53 PM

Like that, yes. The reverse protection diode would connect to the battery. The window used having both a high and low reference 1V apart is what makes the monitoring tricky if using the single supply. The window is the hysteresis.

gmarsh 31st May 2012 05:50 PM

Reverse protection diode can go anywhere in the chain, having it after the voltage regulator puts a diode drop after the regulator which isn't too constant. But for charging a 12V battery, it's probably fine there.

The comparator circuit is pretty easy to pull off. Power the comparator off a 7805/08/09 or whatever's in the junk bin, and divide that voltage with a pair of resistors to establish your reference - connect this to the + terminal of the comparator. Divide the battery voltage with another divider and connect it to the - terminal. Run a hysteresis resistor from comparator output to the + terminal, sized proportionally to the reference divider resistor values to establish the proper hysteresis level.

Feed the comparator output into a logic level NFET (IRLZ24 or whatever, size doesn't really matter) and use this to switch the negative side of the AC relay coil, positive side connected to battery +.

Make sure you fuse the whole circuit. Shorting a lead acid battery can be dangerous.

danielwritesbac 1st June 2012 08:13 AM

Thanks guys!
I don't have an objection to relay logic if it might help avoid a circuitous collection of transistors, but in either case, perhaps a schematic or sketch could make whatever it is a bit more clear.


Originally Posted by Ouroboros (
I guess this is not a Lead-acid battery? If it was then there would be no point in turning the charger off. . .

It is to charge the new style marine batteries. My old charger shoves at it nonstop trying to tip just over 13.5, but she won't go and both the battery and charger gets hot and then you go buy a new battery, again, again. Oh bother.

If left off the charger for 1 hour, or up to 2 days (or more!), the new style marine batteries will all be about 12.6v. I don't see any need for the charger to run nonstop and abuse the battery during those 2 days. I guess that is how the batteries get killed. So, I'd rather the charger turn off--that would at least be more efficient and cooler, wouldn't it?

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