Over time, different batteries will vary slightly in their charge. The 24 volt alternator in a Land Cruiser makes sure that there is always 24 volts, but it doesn't care how that voltage is distributed. Since the system is made of two 12 volt batteries in series, the assumption is that each battery will be receiving an equal charge from the alternator. But, if one battery has a slightly higher charge, say 13 volts, then the other battery will only be receiving 11 volts. This overcharges the one battery, and undercharges the other. If this continues, eventually both batteries will be cooked.
The solution is to use a charge equalizer (battery equalizer). This is a unit that constantly works to ensure that both batteries are equal. It works on a voltage differential - if it senses a voltage difference between the two batteries, then current is directed from the higher battery to the lower battery. As the differential approaches zero, so does the current.
A charge equalizer works especially well if you are tapping 12 volts to run accessories such as a radio, fridge, or laptop. With the vehicle turned off, the accessories can run, draining the 12 volt battery. Meanwhile, the charge equalizer supplements the drain by taking charge from the high side battery to the low side one.
We bought our equalizer from G&S Cruiser Parts. This unit is made by Solar Converters, who offer excellent support. We paid $220 CAD for the 20 amp model (20 amp @ 12 volts).
Installation
There are 4 wires coming out of the box: black (grnd), white (12 volt), and red (24 volt), and an LED light that indicates the internal fuse health. The red one gets connected to the 24 volt terminal of the high side battery (where the starter lead is connected). The white one gets connected to the 12 volt terminal of the low side battery, and the ground is connected to the negative terminal of the low side battery. Connect the wires directly to the battery terminals; do not connect the ground to the frame. The unit needs to see the actual voltages coming directly from the batteries.
There are two internal fuses in the unit. One blows if the amperage going through the equalizer is too high. The other one blows if the ground wire is disconnected from the battery while either of the two hot wires are still connected. Any time I do any work on the truck, and have to disconnect a battery, the hot wires (white and red) must be unplugged from the charge equalizer.
The unit should be installed in the engine compartment at a convenient location. Inevitably the fuses will blow at some point, and you need to be able to remove the top of it to change them. I put mine on my drivers fender apron. I cut the LED wire and lengthened it to run back to the dash. I drilled a small hole to press-fit the light in.
A Note on the LED Light: There are two fuses, but the LED only indicates one of the fuse's health. This fuse is the one that blows if the ground wire is disconnected while the 12v or 24v lead are still connected to the batteries. When the LED light is off, then this fuse is blown. If the other fuse blows due to an overload of amperage there is no indication. If you suspect that it has blown, you can confirm it by turning the unit off, putting a voltmeter across each battery and measuring the voltage. Turn the unit back on, and re-check the voltage. You will likely see a small change with the unit on, and if so then the fuse is ok.
Setting the Voltage
There is a potentiometer inside the equalizer. You need to change the balance of the equalizer by setting the potentiometer. To do this, both batteries must be fully charged.
- Connect the 12v lead to the battery and measure the voltage of the 24v lead to ground. It should be around 24-26 volts. Set the potentiometer to around 25.
- Disconnect the 12v lead, and connect the 24v lead. Measure the 12v lead voltage. It should be half that of the 24v lead. If not, set the potentiometer so that it is.
- You need to iterate this a few times until the 12v lead is half the voltage of the 24v lead.
I want to install a standard car radio/CD player (12v, 45 watts x 4) and 2-small speakers (50-100 watts capacity) What type of Rechargeable battery do you recommend? Also…would I need a regulator? What else do I need for this project? I really need help, thank you.
Chupi, I assume you have a 24 volt vehicle. Can you confirm?
I have a 24 volt trolling motor. How do you conect 2 twelve volt bateries togeather to make a 24 volt system?
Parker, I just wrote this post for you on connecting two 12 volt batteries together, in parallel (for 12 volts) and series (for 24 volts).
Hope that helps.
I am looking for a way of charging two 12V batteries permanently connected in series (for a 24V system) from a 12V alternator.
I believe there must be an electronic device available that connects alternator to both batteries and contineously swaps charging between the batteries so that alternator charges only one battery at a time (for a fraction of a second to several seconds at a timne).
If there is no off the shelf device, can you advise a simple cirquit?
Thank you
we have a moterhome and want to hook up 3 12volt batteries to run everything so we don’t have to start generator much. could this be a fire hazzard? will it give more power than we need if hooked to axillary battery?