Can I build my own 12V battery charger with this?
 

My existing "smart" battery charger won't turn on if the battery is completely flat, and it will shut off if the car is drawing too much power. Well, I plan to run my BMS and a fan while grid charging my high voltage battery all night, and I'd like to recharge my 12V battery at the same time.

I could get a MeanWell RS-50-12 (for $23!) which can supply 4.2A @ 13.2V. However, if you ask for more power than that, as battery charging would do, it goes into hiccup mode, shutting down then powering back up. No good for powering my BMS.

http://www.epanorama.net/zen_schemat...ower/basic.png

If I add the current limiter illustrated here, do I then have a cheap, fast 12V float charger that doubles as a 12V power supply?

How low is your 12V battery getting? Deep discharges are bad for batteries not designed for that kind of use.

A circuit like that can work but you'd need to beef it up to do any good.
Probably better and certainly less time to get a less smart car battery charger that doesn't care what the battery voltage is.

If you do want to use the Meanwell, you'll need to set it higher than +12V output not just due to the headroom of your circuit but also that lead acid batteries need to be charged higher than 12V (like 13.8) and occasionally need to be equalized by charging them to an even higher voltage for a bit.

What you have drawn is a 70mA current sink. If you place that in series with the battery then charge current will be limited to 70mA which seems a bit low.

If you are using a conventional power supply as a battery charger then you want to set the output voltage just above the battery voltage. Then raise the power supply voltage while monitoring the current (to stay out of the protective hiccup mode). Come back every hour or so to raise the voltage in order to maintain charge current. Stop raising the voltage at 13.8V or so.

I needed a 12V*1A power supply to run my computers and battery fan anyway, so instead I ordered the 4A model, and we'll see if I can get it to charge my 12V too.

It is a deep cycle battery, rated for 1000 cycles (4yrs here) to 50% capacity at 50% DoD, so I try not to discharge it too deeply. The datasheet says I can float charge it at 13.1V indefinitely, and I could do that with this power supply.

The float voltage changes with temperature and better chargers will adjust based on temp. I remember 13.8V being the safe voltage at room temp (25 deg C I'm guessing)

One can never have enough power supplies. Batteries and power supplies are my vice. Came across a good deal on 12V 1KW power supplies on Ebay a few years ago. Still haven't used them yet :p

Edit: Did some googling. 13.8V is too high for batteries that are seldom discharged. Here's a good page on it. Also talks about equalization charges.
http://www.solarnavigator.net/battery_charging.htm