For my camper, I want to charge several 12V batteries independently.
I've looked at car battery chargers, but they are incredibly expensive ($50-$300).
I understand that they have some circuitry which monitors current, voltage, and has different settings for different phases, to optimize charging and battery life.
I am wondering what makes them so expensive. It seems to me that they are little more than converting input voltage to 14.4V, then applying PWM, constant-current etc.
What am I missing?
Bonus question: Can I take a PC PSU 12V 40A, boost it to 13.8V, and connect that to my battery? (And then use Arduino to monitor the voltage, and to disconnect when done with charging etc.)
I think $50 is exceedingly cheap for a good charger. There is considerable design work in them.
I would be happy to pay a couple of hundred to save the large amount of time and parts required to make a charger anywhere near as capable as the commercial ones.
I have charger which I used on my boat and which cost something like $400 Canadian but it's cadilac.
Starting with a dead battery it charges at constant current (its 20A rating). This results in the battery voltage slowly rising. The charger shuts off to check the voltage every few minutes and when the voltage gets up to a preset level the charger changes to constant voltage. This results in the current slowly falling and when it gets down to a low preset current the charger switches to a low trickle current which it maintains forever.
This works extremely well, gets the battery as fully charged as possible and gives a very long battery life.
So there you have a good working algorithm and you should have a lot of fun implementing it. Whether or not you save a lot of money is another matter.
Most of the modern battery chargers use a switch mode power supply which is quite heavy duty and have a Micro
to control the charge rates.
Most will do trickle,bulk and equalization charges.
Theres a lot in them if you pull one apart.
You can modify a PC power supply to make it a battery charger, BUT you have to really know what you are doing.
You will need a schematic of the power supply as you will have to modify the 12V feedback voltage, and you will have to defeat the 12V over voltage protection circuit.
THIS IS NOT A PROJECT FOR A BEGINNER.
I was thinking to simply buy a cheap boost converter from ebay, with current limiter, for $5, and boost the 12V to 14.4V, with a limit of 10A. I imagine that the boost converter would reduce the voltage and limit the current to 10A in the first phase, then slowly increase the voltage until 14.4V, while the current would naturally be falling to lower levels.
And perhaps add an Arduino controller to monitor battery voltage, temperature and perhaps charge current, with a relay/mosfet to disconnect charging/do pulse charging.
The whole setup would cost only $30.
But you are saying that it simple. What am I missing? Why is it not simple?
A solar panel is anything but 12v.
The rating normally nominally means it can charge a 12 V LA battery.
Off load voltage output can be anything from 0 to 20 volts or more.
With correct sizing the battery does the regulation for you provided you use a shunt zener of 14 or so to prevent overcharging.
In the original post I wrote "Can I take a PC PSU 12V 40A..".
I understand the confusion. I have solar panels, but if my batteries run low, I want to disconnect them and charge them from grid power instead. I have three batteries, each of which I want to charge with no more than 10A (because I was told that that's the healthiest for them). Basically a PC PSU can provide 12V 40A, which could then power 3 boost converters at 14.4V 10A.
This would be a lot cheaper than having three seperate battery chargers. Not to mention that I could monitor battery condition etc, which wouldn't be possible when parallel charging at 30A etc.
Your 10A number is pretty conservative. Basically you can charge a dead lead acid battery with quite high current but as the voltage comes up you must taper back on the current. The real limiting factors are temperature and gassing. If the battery temperature goes too high you have too much current. Also if the battery starts gassing too much the current should be reduced otherwise it uses up the water and if you don't back off on current the lead plates will be exposed to the air. This permanently reduces capacity if it goes on too long.
Your setup, as you describe it is much the same as I used on the boat and the charger I described above did a very good job. I also had a small solar panel which, as far as I could see had no regulator of any sort but at least kept a trickle charge going.
Any idea why the chargers are so expensive though?
If the pricing is mainly justified by complex software algorithms?
It doesn't seem to me that the hardware is that complex? Especially since the output 13.8V is allowed to be dirty...
The voltage of the supply you use to charge the battery and the float voltage of the battery are both temperature dependent for lead acid. Make sure you take the time to look up what those voltages should be.
Right. I've read that many times as well... yet I've never seen a charger where a temperature sensor was actually mounted on or near the battery, despite that such sensors would be very cheap.
re tutuli's comments on PC psu's - they may be rated 500W, but the 12v output certainly
won't supply 40A at 12v - probably only 2. Most of the power is delivered at the lower voltages.
As to controlling solar panel charging - not quite trivial. A clever one deals with the variable impedance and voltage of the cells, and uses a boost convertor at low light levels. As the light goes up, it swaps to buck, and limits the voltage to about 14. I've designed something similar for a small outboard motor charging coil.
A simple way to control overcharging is a Zener - but it's ghastly. And decent solar array can put out hundreds of watts - and you'd have to dissipate it - not so easy!
Old brit motobikes used this method to limit the output of their permanent magnet alternators. euggh!
A better simple way is to use a switch (a big MOSFET )with a voltage sensor with a little hysteresis to switch it ie it charges to 14.4, turns off, then when the battery volts drops to (say) 14, turns on again.
allanhurst:
A simple way to control overcharging is a Zener - but it's ghastly. And decent solar array can put out hundreds of watts - and you'd have to dissipate it - not so easy!
There was a discussion about this a while back.
I came to the conclusion that MPPT controllers were only economically viable for panels of a few hundred watts upwards.
Most of the questions here are about phone chargers and the like.
My 50 watt panels use a shunt regulator but its a bit more than a zener.