Adding DC-DC charger to existing Solar + Split Charge system

[db123]

Hi guys,

I've been reading through threads but cant find the answer I'm after.
I currently have 3x 50W monocrystalline panels connected in parallel to a 75/15 MPPT controller to my ~120Ah AGM battery. I also have a split charge relay between starter and leisure batteries. Recently while staying at a friends house I found myself a bit stuck when I accidentally left something on in the front cab overnight (late evening cocktails in the sun may bear some responsiblity). My starter battery was totally drained to about 2V. Luckily all I had to do was unplug my solar output from my mppt to the starter battery and wait 2 days for the starter to charge.

In order to do this I had to take apart my electric rig box to get to the cables to switch them over. I'm assuming this isn't best practice because the history of the battery in the mppt will get confused but I've been living on the road for years and it's the first time I've drained my starter accidentally. I'm thinking of adding in a way of switching from solar-output-to-leisure to solar-output-to-starter, if and when this happens again. My first thought was, just get a switch that will let me transfer the solar output from one battery to the other, maybe clear the history to avoid mppt confusions and then switch it back when the starter is charged enough. The alternative is to add a DC-DC charger between leisure and starter battery but I'm totally confused on the best way to do this.

I don't think I need a £150-200 dc-dc charger when a basic voltronic with a switch will probably be sufficient but Iknow very little about balancing, battery drains and the relationship between various power sources. I guess my question is, are any of these bad setups, is there a better way to go about it:

1) Add a votronic 3065 with an isolator switch between the leisure battery and the starter. If I need to charge my starter, flip the switch, let it charge and then disconnect it after.
2) Add a DC-DC charger between my leisure battery and my starter battery. Would I be better connecting the solar to the starter or leave it connected to leisure? Is it ok to leave the split charge relay in there?
3) Use a three position switch for the solar output from the mppt. It will stay on leisure most of the time but I can also switch it off or switch it to starter battery. I'll reset the mppt each time I have to use the switch but it's not going to be used more than once a year at most, so I'm sure the batteries and mppt won't stop being friends, right?

Any help or advice would be really appreciated!
All the best,
db123

 

[12VoltInstalls]

You may be over complicating this.

For starters I’d suggest isolating into separate systems everything you might use while parked- including radio, lights, etc. Over many years of boat work I have always kept the starting battery holy- always had a separate battery (batteries) for accessories, radio, vhf, etc.
In your case I’d go so far as installing a second car stereo head unit (so you’re never tempted!) wired to the solar power system (you can use existing speakers) and disconnect courtesy lighting from the vehicle system, also wiring it to your onboard house power.

Now that that’s out of the way, a simple high-amp switch like a marine A/both/B battery switch can be installed to charge the starting battery. But keep in mind that a) separate systems should keep you from requiring that, and b) THAT is the point of failure if you experience another Operator Head-Space Error if you are unwise enough to switch it to the starter battery.

 

[db123]

Oo interesting. Not sure how much I trust my ability to reroute the cab lights, radio and cigarette lighter (the only offenders as far as I know) to the leisure battery but definitely a good solution. Everything in the back is isolated already so it shouldn't be too tricky. I feel like a backup solar to starter would still be good just in case.
I usually carry a jump starter pack with me but left it with a friend on this trip - "ah don't worry. Keep hold of it mate, I won't run out of battery!"

 

[icyroads]

If the battery chemistries are the same, and capacities are not too different, just use a VSR (Voltage Sensitive Relay) to charge both, prioritizing either the starting battery or the house battery according to your preference.
Some VSRs have an “override” switch that you can hook up to a push button switch (or any other switch or signal) to engage the relay anytime you want. Do be mindful of the current that may end up flowing through the switch if one or the other battery(ies) are depleted (I read that it’s not really a problem with lead acid batteries, but I still suspect it could be with a lot of batteries in parallel), and also the effect upon the alternator (which could experience a big empty battery bank as an effective short).

 

[icyroads]

Oo interesting. Not sure how much I trust my ability to reroute the cab lights, radio and cigarette lighter (the only offenders as far as I know) to the leisure battery but definitely a good solution. Everything in the back is isolated already so it shouldn't be too tricky. I feel like a backup solar to starter would still be good just in case.
I usually carry a jump starter pack with me but left it with a friend on this trip - "ah don't worry. Keep hold of it mate, I won't run out of battery!"

FWIW, I keep on hand one of those capacitor jump starting devices. They have no battery at all, just a giant capacitor(s), and they’re comparatively much smaller and lighter. They have upvoltage converters that can charge even from an apparently “dead” battery, or any other battery or DC power source.

 

[camelCase]

I like the 3 way marine switch idea. I'd also recommend a jump pack.

Amazon.com : lithium jump pack

 

[mikefitz]

1) the Votronic 3065 is a maintaining charger that will transfer a small amount of power from your leisure, house, battery when the house battery in under charge. It effectively connects the two batteries together when the house battery is 0.6 volts higher than the starter battery. The maximum current is limited to few amps. Its designed to keep the engine battery In good condition . Its not designed as a charger.
If you had this In circuit when you discharged the engine battery, the house battery would also have been discharged. Its not a very smart design .

2) if you have a split charge in circuit then normally this would have to be disconnected if a B2B is installed.
You have a split charge Installed? If so what model?
Using a B2B to transfer power from house to starter battery in an emergency is expensive and will take some time.
A switch to connect the two batteries with suitable cable and fuses is a reliable and inexpensive option.

3)connecting the solar to the charger is an option but a simple alternative is to leave the solar connected to the house battery and fit a 'smart' split charge unit, the Victron Cyrix combiner with start assist.
Its an Inexpensive unit and,
Will charge the house battery when the engine is running.
Will transfer charge from the house to starter battery when the house battery is full.
Offers a start assist, via an additional switch, will activate the unit and connect the batteries together.



Mike