I am helping a friend with enhancing his camper van. Situation is as follows: He may go camping for a 2weeks and want to use car grid to power lights, charge cameras, phones etc. But don't want to risk discharging battery too much(he need to start big diesel)

My idea is to wire second(utility) battery(in the back of the car) parallel to main battery. But not directly. I plan on using switches or relays to disconnect the utility battery when voltage gets below certain point. That will enable him to use remaining power in utility and ensures he can start a car. Then comes the charging/balancing the batteries. To limit balancing currents resistor(beefy one) will be placed between batteries until difference is low enough to connect them directly again.

With help of simple arduino whole system can be totally "set it and forget it". Is it really that simple or did I miss something important?

EDIT: According to responses I probably badly described what worries me. Its connecting batteries in different state of charge. I am not very familiar with lead-acid, but from my knowledge of LiPo you cannot simple connect them and let them balance on their own because you will damage more discharged battery by charging it too quickly. It also generate lot of heat in the proces and currents are tremendous. Hence the resistor between utility and main bat after discharging one bat more than the other.

  • \$\begingroup\$ You'll need some really beefy cables, as well. Those batteries can output a lot of current. \$\endgroup\$ – Hearth Apr 26 '17 at 12:48
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    \$\begingroup\$ Avoid the use of dissipation elements like resistors. Also, you will probably want a charge controller to ensure that both batteries charge or to control which battery charges first. An MCU and some relays can do, however... the most reliable method is going to be to not use the auto batteries and only the utility battery for non-starting loads. (e.g. unless you use a deep cycle battery for your starting system... leave it out of the 'power random things' game) \$\endgroup\$ – david1024 Apr 26 '17 at 13:39
  • \$\begingroup\$ There are commercially available products to do just what you describe if you plan to do it often. A beefy schottky diode would be the simplest solution if you can live with the 0.4 V drop to ensure you can never drain your starter battery by leisure loads. Also, a small $100 li-ion jump starter will start your car/truck with no sweat even with a flat battery. \$\endgroup\$ – winny Apr 26 '17 at 13:44
  • \$\begingroup\$ Yep, beefy cables are must. Main problem is balancing. Using heavy resistor to create voltage drop and limit current seemed like cheap and relatively reliable solution to roughly balance them. Before connecting them directly again. \$\endgroup\$ – Seth Rayer Apr 26 '17 at 14:00
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    \$\begingroup\$ fwiw, lead acid batteries are a lot less finicky than Li when it comes to mixing and matching. not saying mixed charges are optimal, but they won't explode and kill you... \$\endgroup\$ – dandavis Apr 26 '17 at 20:16

You can buy split charge relays or controllers that come in many different types and sizes.

You can buy a simple split charge relay that has voltage sensing circuitry that charges both batteries and will keep both batteries separate so the starter battery never gets discharged below 12v. These can be found as cheap as £/$10

You can buy more sophisticated micro-controlled systems that monitor both batteries and charges each and switches between the two, they also often have inputs and outputs for extra charging sources such as extra alternators and solar panels. The can also have dedicated outputs for critical equipment such as fridges, communication/GPS devices etc. These range in price from £/$50-1000 depending on the spec.

You could also use a DC to DC charger to maintain the second battery this would be required in some modern vehicles with 'smart alternators'.

  • \$\begingroup\$ I didn't know about split charge relays. Thanks for hint I will definitelly research on this topic more! From quick glance on schematics and pictures on google images I see it can divide batteries similarly to what I planned. But what after disbalance?(one more discharged bat than the other) Will it simply snap them back to parallel and let the batteries balance on their own? \$\endgroup\$ – Seth Rayer Apr 26 '17 at 14:34
  • \$\begingroup\$ @SethRayer the intention of a charge controller is that the vehicle battery and the utility battery are discharged separately, but charged together. You don't want to run the utility battery flat overnight, then accidentally run the vehicle battery flat attempting to equalise the charges - because you may not be able to start the vehicle again. \$\endgroup\$ – Simon B Apr 26 '17 at 14:43
  • \$\begingroup\$ @SimonB of course you will connect batteries back together only than alternator or another charing circuitry is running. There is no reason for balancing/equalising them without power source, this way there would be no reason to disconnect them in a first place. \$\endgroup\$ – Seth Rayer Apr 26 '17 at 14:49

You should use a split charge relay, so when the car is running the secondary battery charges from the alternator (and both batteries would be used for any phone charging). When the car is not running only the secondary is used. Unless you're charging 1000's of phones and running a kettle you are unlikely to fully discharge a car battery. And even if you do you can just switch the engine on and let it charge. Like others have said, use some thick cable.


You don't need anything sophisticated unless you have a healthy budget or uber R&D skills. BUT you do need to measure your battery capacities.

How will you know much margin you have to start the diesel with any SoC of any Battery design?

What is the consequence of failure?

How will you connect the two batteries with a known CCA rating but unknown battery health and SoC? What is the max surge current expected if one battery is charged 100%SoC and other one dead at 0% SoC?!?

Since batteries have a low ESR like that of the starter DCR you can expect similar surge currents on connecting them with different SoC voltages.

CA and CCA ratings are done at 7.5V while Ah ratings are done at a 20 h rate. ESR of battery rises sharply as battery drops below 10%SoC thus voltage during "cold start" drops more with low ; SoC, battery health, age, temperature , dry cells and low s.g.

An inexpensive s.g. tester for cell acid is advised too for testing mismatch % in each cell as a battery health and SoC indicator. New is <0.1% matched , while Dead , shorted , or sulphated can rise to >10% mismatch.

There are many solutions to each problem using dual diodes, starter solenoids, smaller transfer relays to bypass the active solenoid so they can be shutoff and only used to connect and disconnect the batteries.

You need a few tools, specs and skill to do this right; plus an emergency solar panel or generator source like a motorcycle and current limiter.

A cheap DVM may be powered by the car battery with a 9V LDO to monitor both batteries.

You need to estimate Ah SoC using a Voc chart after a short term load or long wait and you can test each battery for Voc vs SoC, normally 12.5 to 11.5 for 100% to 1% SoC. Then measure battery Ah and voltage during battery drain over a 4 to 8h period and expect 10 more for a 40 to 80hr period.

THEN plot this Ah consumed vs V and Voc for each battery.

motor start at various levels from 10% for starter reliability then charge it by adding 10% of actual Ah capacity (not just rated)

You can simply use a DMM and keep both batteries in tandem and use rear access for camping DC outlets, with jumper bridge cables or smaller gauge wire based on a safety breaker rating that you add-on. Then you test and decide how much margin you need between charging.

50% SoC is near 12.0V as a rule of thumb but you battery health may affect this due to a weak cell.

20% SoC may be a safe lower limit depending on your rig.

A small Solar Panel may be a safety net.

A typical van battery "might " be around 50 Ah or more when new.

Keep in mind Relays are rated for much lower loads in DC than AC but Automotive Relays are cheap and will conduct higher current if not switching it.

Starter solenoids may be used to connect batteries momentarily then bypassed with a lower Amp rating Relay or suitable switch.

Automation is the easy part, but testing and design specs are the hard and necessary part for reliability.

  • \$\begingroup\$ Thanks for comprehensive answer. To clarify, main battery would be disconnected from utility grid rather early based simply on voltage. Than user can milk utility battery until it is dangerously low. After that he is either out of juice or have to start car and recharge them. As I mentioned in edit to my question. Connecting the pack back together is my concern since I imageine higher SoC battery wil try to balance with lower SoC bat and high currents will flow possibly damaging charged battery or relays. \$\endgroup\$ – Seth Rayer Apr 26 '17 at 14:29
  • \$\begingroup\$ using an 800A car starter solenoid to connect/discconnect batteries for a few seconds then bypass with a 50A relay will work, for example as I suggested will work. Size according to specs needed. A light bulb (headlamp ) can also be used between batteries to charge the weaker one. with a 15A relay then bypass relay across bulb when dim turns to off. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 27 '17 at 2:27

I've seen these switches in motor boats where you want to use one battery to run a trolling motor (say) and keep the other for starting. Search for marine battery selector switch.


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