What is in a battery equalizer (lead acid) that would make it so expensive?

[KTP]

If you have a 24V battery system and need 12V for some equipment, one method is to use what is called a battery equalizer.

It looks like all the device does is monitor the series connected batteries and alternate which battery it is tapping power...thus it can operate at a high efficiency because it does no DC-DC conversion.

The main ones I find are from Vanner and run $500 to $700 on the low end.

What is inside these more complicated than a 8 bit micro, a few power devices and some smoothing caps?

[Lightages]

Three reasons:

1. Very very low production volumes does not make low prices
2. This is potentially switching very high currents and the power devices need to be very robust and well heat sinked
3. The switching has to happen very very very quickly without interruption to the load and therefore the designshould be super critical

[KTP]

I will have to run some LTspice simulations to see how quickly and how sharp it really needs to switch in order to have minimal ripple and spikes.  I am not 100% convinced it can't be done simple and cheap.

[NiHaoMike]

How much current do you need? A buck converter would probably be the way to go.

You might want to check with alternative energy DIYers. It's very common to start with a 12V system, then upgrade to a 24V or 48V main battery bank later on.

[KTP]

How much current do you need? A buck converter would probably be the way to go.

You might want to check with alternative energy DIYers. It's very common to start with a 12V system, then upgrade to a 24V or 48V main battery bank later on.

12V accessories, furnace fan, window fan, some LED lighting.

I would say no more than 30 amps at 12V.  45 amps would be nice for a short period.

I know I could run a DC-DC off the 24V to give me 12V, but the really efficient ones are not really cheap at that power level.  I have some nice Vicor modules but they are nowhere near 30 amps.   Plus most of the DC-DC converters have a pretty large standby current draw.

[ConKbot]

I think that the switched tapping thing might not be correct, as one battery is grounded, so its not like you could alternate using one battery than the other unless all the loads were floating/non-grounded. Which could be a poor assumption with car accessories.  With plenty of fault current available for when something is grounded :p

On a  related side note, I had a DAQ system which rode along in a vehicle that was being tested.  We had switched power provided to us by the vendor, and we installed our system, gauges, and the antenna for the wireless system.  We got a call a few days later that the wiring harness to power the DAQ, and the antenna wire had melted and started smoking... after they shut off master power.  For some reason they manufacturer decided that it was a better idea to switch the negative lead on everything rather than the positive.   Our magnetic antenna had been moved from a painted surface to an unpainted bolt, and when they switched off the power, everything that was switched off tried to flow through our 18 AWG power wire, though our radio and down the RG-58 to the antenna base.  The wire was pretty melty, and the RG-58 had been red-hot because the copper shield braid was crunchy too.  The fuse for our DAQ was on the positive wire, so that was of no help.  So yeah, dont do floating ground systems in vehicles :p


I would imagine that the equalizer isnt anything fancy though, a vicor module with its isolation would be overkill.   A simple current mode chopper, and a few voltage dividers to feed a uC.   The chopper is just an inductor, FET, freewheeling diode and current shunt and shunt amplifier. Turn the fet on, when the current hits a limit (shunt + amp + comparator) shut off the fet.  Have some hysteresis on the comparator so the current drops and the fet turns back on. So it would make a simple buck-converter to step down 24V to 12V.   Have the micro run the the comparator when the batteries are imbalanced, and when the main system voltage is >26 VDC. When its under 26VDC, put it in sleep mode since the vehicle is off. 

You could really do it all with a quad comparator, no need for a uC (current > threshold? Vin > 26V? top battery > bottom battery? And a free comparator for something else, alarm if the bottom battery drops too much because it cant keep up? )   But either way its a chip and a bunch of resistors, so if you dont mind the programming, call it about the same either way :p )

Doing it this way you could surge as much current as you need since youre hooked directly to the 12v battery, and the chopper is just charging the battery from the other one. As long as the chopper can keep up in the long term, it will stay balanced.

[Pack34]

Do you have the ability to switch to 2 12V batteries and then run your system as an unbalanced load?