Simple Dual Car Battery Charger

[Matttcattt]

i have built a gainclone amp to run off +/-12v, to go in my car, when i get one.

in the mean time, i want to run the amp off 2 12v car batteries. i want to build a simple dual charger, with circuitry to automatically turn the chrager on and off.

i found these circuits, the first is a circuit using the L200 variable voltage regulator, rated at 2A. is this suitable for car batteries?

the second is a circuit by rod elliot which switches the chragher on and off. does anyone have a better/simpler circuit?

my last question concerns the power supply. i have a dual secondary, 25v, 60VA toroid. the ~35vDC i should get from this is ok for the L200, but the heat dessipation will be huge. i was thinking of putting the secondaries in parrallel to get ~17.5v, which is about what i need. this will only leave me with one supply, between the 2 chargers, which means that the 2 batteries have to be seperated from the aplifier before chrarging. whcih method should i use?

[crown300]

Matttcattt,

Please note that car batteries are VERY dangerious. This is not something to play with.
1) Incorrect charging of a Led Acid battery may cause it to explode. Boom. Note what flies out of the battery = ACID.
2) Car batteries give off hydrogen gas when charging. Yes, you have a potential Hindenberg here, just add one small spark.
3) The acid which can easly leak out of the tops can disolve your clothes (and skin).

If you must use batteries, use (either) "sealed Lead Acid" or "gel cell" type batteries with the correct charger for that type of battery. Use of the wrong charger can ruin the batteries or worse.

[Matttcattt]

Originally posted by crown300
Matttcattt,

Please note that car batteries are VERY dangerious. This is not something to play with.
1) Incorrect charging of a Led Acid battery may cause it to explode. Boom. Note what flies out of the battery = ACID.
2) Car batteries give off hydrogen gas when charging. Yes, you have a potential Hindenberg here, just add one small spark.
3) The acid which can easly leak out of the tops can disolve your clothes (and skin).

If you must use batteries, use (either) "sealed Lead Acid" or "gel cell" type batteries with the correct charger for that type of battery. Use of the wrong charger can ruin the batteries or worse.

i realise how dangerous car batteries are, but as i am going to have to buy two when i put the amp in my car, i though i would buy them now, and use them, rather than building a huge power supply. a PSU would cost ~£100, and i dont have this money. i have ~£50, which will get me 2 car batteries.

do you think i should buy a charger (or two), instead of making one, or are you 100% against the use of car batteries?

also, safety warning (as per forum rules):

######################################
# WARNING: CAR BATTERIES CAN SUPPLY LARGE AMOUNTS #
# OF CURRENT, AND THERFORE, ARE DANGEROUS. #
# I advise you not to touch car batteries, unless you #
# understand the dangers, and know what you are doing. #
######################################

[crown300]

I am 100% aginst using "car" batteries - just too dangerous.

There are better solutions out there.......

[Immo_G]

I think most car chargers are around 5 - 6 amps, i would just buy a cheap charger or 2.

[Circlotron]

Originally posted by Matttcattt
I have a dual secondary, 25v, 60VA toroid. the ~35vDC i should get from this is ok for the L200, but the heat dissipation will be huge. i was thinking of putting the secondaries in parrallel to get ~17.5v, which is about what i need. this will only leave me with one supply, between the 2 chargers, which means that the 2 batteries have to be seperated from the aplifier before chrarging. whcih method should i use?

Leave the two batteries in series and charge them to a total of 28.4 volts. (14.2+14.2 volts). 35v regulated down 28.4v is a bit nicer for your regulator than down to 14.2v.

BTW, car batteries in particular the low maintenance kind have a somewhat different chemistry than the sealed and gel cell type e.g. they have more antimony in the lead grid to lower water loss. Translates into higher charging voltage. Sealed and gel types usually charge at 13.65 volts.

[Matttcattt]

Originally posted by Circlotron

Leave the two batteries in series and charge them to a total of 28.4 volts. (14.2+14.2 volts). 35v regulated down 28.4v is a bit nicer for your regulator than down to 14.2v.

BTW, car batteries in particular the low maintenance kind have a somewhat different chemistry than the sealed and gel cell type e.g. they have more antimony in the lead grid to lower water loss. Translates into higher charging voltage. Sealed and gel types usually charge at 13.65 volts.

is it safe/a good idea to chrage both in series? i had not though of this. thanks for the idea...

[Circlotron]

Provided they start off at an equal state of charge and are both the same capacity and condition / age then yes. In telephone exchanges they use 24 cell batteries (48V) like this all the time. Float voltage is 54.6v.

[Matttcattt]

would 2A (the limit on the chip) be enough to charge two car batteries in series? or should i use a transistor/s to increase the current capacity, as per the datasheet?

[Circlotron]

If the car battery has a capacity of 40 ampere hours (fairly common size for ~3 litre engine) then 2 amps will charge it in about 20 hours.

Actually, it's not a good idea to deep discharge car batteries because the lead paste in the squares of the pure lead grid swells up and when it contracts upon recharge it pulls away from the grid and ceases to make electrical contact so the battery capacity gradually decreases till it fails. OTOH, car batteries are dirt cheap compared to deep cycle ones so just do whatever you like and big deal!