Dual battery bank, Split charging, Voltage sensitive relays - Page 1
Forum section: electronics / general

author: StokesPen
author_rank: Newbie
timestamp: June 21, 2017, 04:52:59 pm
content: I've got a question about dual battery setups. Let's say it's in a boat with a 12v battery to start the engine and then a larger 12v bank for use when the engine is off.You can use a voltage sensitive relay or manual isolator to ensure the battery for the engine isn't drained.Overnight the "leisure" battery is depleted. There is now a large differential in voltage, the engine battery is at 13.2v and the leisure battery 12.2v.When the engine is started and the connection is made joining the batteries together again, my understanding is the batteries will basically be directly connected in parallel and desperately trying to equalise voltage, this could draw hundreds of amps from the engine battery and damage it along with pushing to high current into the leisure battery? Of course the alternator will be chugging along pushing 100 amps or whatever so both batteries will end up charged but my question is this:Is there anything to stop this high initial current draw, am I missing something? is it as bad as I think it is? is there a current regulator device for such setups?Thanks

author: SeanB
author_rank: Super Contributor
timestamp: June 21, 2017, 06:16:37 pm
content: Typically you use a split charge controller, which is basically just 2 very beefy diodes, so the alternator charges both batteries in parallel, though the charge is going in the majority of time to the battery with lowest voltage. Diodes keep the one battery from charging the other, but the alternator has to provide the current. Yes if the one battery is flat the alternator will be running at maximum current, which in many cases will be bad for a modern cost cutted alternator, or one that is controlled by an engine management unit that is only using the engine battery as reference, and in many cases you will probably want an aftermarket alternator or a second one for the auxiliary battery.

author: StokesPen
author_rank: Newbie
timestamp: June 21, 2017, 06:56:26 pm
content: ahhh, I'm starting to see, the diodes make a lot of sense and solves the big problem I was thinking of. Just the mention of it has enabled me to search and find more relevant information so thanks for that.

author: Codebird
author_rank: Regular Contributor
timestamp: June 22, 2017, 09:30:35 am
content: There is another issue to be aware of: Diode drop. Even the best schottky is going to be at least 0.2V, and if you are floating your engine battery at 13.6V and your leisure battery at 13.4V... well, one of those isn't going to be spot-on ideal. It'll work, it'll just shorten your battery life a little bit. Maybe you'll have to replace it in four years rather than five.There is a simple solution to this: Use an ideal diode circuit.

author: Nauris
author_rank: Regular Contributor
timestamp: June 24, 2017, 06:57:41 pm
content: Quote from: StokesPen on June 21, 2017, 04:52:59 pmNo, it is not that bad. There is resistances in wiring and in the batteries so 1 volt difference is not going to make too big currents.

author: Inverted18650
author_rank: Frequent Contributor
timestamp: June 26, 2017, 12:42:56 pm
content: If you have easy access to the batteries, maybe run a little experiment. Once they are discharged under their normal conditions, remove both of them. Then hook them up in parallel with an amp meter on the high side. Place the positive lead of the Amp meter on the pos terminal of the battery with the highest voltage and the negative lead of the Amp meter on the lower V battery. (Maybe place a 10Amp fuse inline as well). As soon as you make the last connection, you'll see the exact Amp draw being transferred from the high V to the other, as they balance out.Note: I chose a 10A inline fuse because most off the shelf multi-meters will only test up to 10A's anyway. Put the fuse before your DMM on the + side, this way the inline fuse will blow before any current gets to your DMM. Like Naruis said above, since the differential is only 1V, I think you'll find the current won't be very high during the transfer. The exact number is dependent upon the batteries ESR, but unless its a crazy high Ah battery, I think you'll find the energy transfer is far less dramatic. Also, when the boat relay triggers, you have an external source, your alternator, providing current, which means even less has to come from battery A.

author: Ian.M
author_rank: Super Contributor
timestamp: June 26, 2017, 01:27:42 pm
content: For any reasonable capacity battery bank that sounds like a good way of popping the $expen$ive$ HRC fuse in your DMM, (or burning out a cheap DMM). However it is in principle a worthwhile test - if you isolate the battery positive terminals either by removing the connectors or using the existing isolator switch(es) then use aDC rated clamp on ammeterround a jump lead to parallel them and see the peak current draw.The smarter battery management systems using MOSFET based switching monitor the current through each MOSFET, and if it exceeds a safe level, quickly turn off the MOSFET to protect it.If you were designing your own battery combiner system, you could start with a pair of beefy Schottky diodes, that would allow the Alternator to bring the batteries up to a similar terminal voltage, and also have a relay system that bypasses (shorts out) both diodes if the engine is running and the battery voltages have been near enough the same for several minutes. That allows the alternator to bring the house bank up to a reasonable state of charge before cross-connecting them.However, unless you have a very high capacity house bank, anything so complex is probably overkill, and if you do have a very large house bank, its probably simpler and better to have an extra high output alternator for it and leave the 'stock' charging system only feeding the engine battery.For many years, I have been using a simple system that taps the alternator charging indicator light for a control signal to drive a 80A relay (protected by a 70A fuse) that simply parallels the engine start and house batteries. The alternator is permanently connected to the house battery as that's the one that needs the most charging. In use its entirely simple - turn on both isolators when you first go aboard, and only turn them off if you are leaving the boat overnight or longer. There is also a manual paralleling switch that activates the relay so instruments and other systems can be powered off the engine battery if the house battery is failing. The failing battery can be isolated with its isolating switch if its got a shorted cell and is dragging the voltage down. If you are dumb and try to use it to start the engine off the house battery, it *WILL* blow the 70A fuse. In the engine toolbox, I keep a pre-made high current jumper to link the two isolator switch output terminals for that scenario.

author: StokesPen
author_rank: Newbie
timestamp: June 27, 2017, 06:47:04 pm
content: Very interesting ideas Ian, thanks

author: batteksystem
author_rank: Regular Contributor
timestamp: October 24, 2017, 08:36:19 am
content: That is actually what I am selling on ebay: Voltage Sensitive Relay that detect the voltage at the battery terminal to determine whether the alternator is running or not in order to decided whether to close / open the electrical connections between two batteries. The closing voltage is 13.3V while the opening voltage is 12.8V (a 0.5V hysteresis).