Keith Tanner
Keith Tanner MegaDork
7/14/17 12:14 a.m.

A Westfalia counts as an RV, right?

I've got a solar setup on our little van which works a treat. It's connected directly to a pair of matched house batteries and seems to be doing a great job of keeping them happy. When the van starts, a solenoid connects the house batteries to the alternator so they can charge. This seems to be working pretty well.

But the starter battery is getting cranky about the long waits between trips and I've had to put it on the charger a couple of times. I'd like to be able to maintain a charge in it using the solar. What's the best way to accomplish this?

A friend suggested a battery isolator using the solar as the "alternator" power source. I want to make sure I can't drain the starter battery if the house batteries get depleted. I also don't have burly enough wiring to use the house batteries as starter batteries, so we don't want them trying to start the car.


¯\_(ツ)_/¯ SuperDork
7/14/17 6:34 a.m.

I think your friend's suggestion is correct, at least for the charging portion. I've also heard these sorts of things referred to as a "blocking diode." If you really want to be certain that the house batteries won't try to help the starter, you could add another relay that cuts them off completely when the starter relay is powered.

wae Dork
7/14/17 6:35 a.m.

Most setups that I've seen have a solenoid that is used to link the two battery systems. Sometimes they call it an emergency start, sometimes it's called dual batteries, but check to see if that isn't already implemented.

If not, you can put that solenoid in which essentially will connect the two positive terminals since it should already have a common ground between the two systems. Wire that to a switch on the dash and you can switch it on to charge both sets of batteries from the converter if on generator or shore power or from the solar charger. This one from Amazon is a little pricey but will basically do the work for you, connecting the batteries once a charging source is detected. Either way you go, it will also allow the house batteries to charge from the alternator while you're driving.

drsmooth HalfDork
7/14/17 9:15 a.m.

On my boat. I have a isolator to keep the house batteries and the starter battery separate. When depleted, I can flip the switch to charge the house batteries from the alternator. If your house batteries are flooded lead acid deep cycle keep in mind that they don't usually like being charged with a alternator. So the lifespan may be reduced. I overcame that problem by switching to a absorbed glass mat deep cycle. AGM deep cycle batteries don't seem to mind the abuse as much as flooded lead acid.

Keith Tanner
Keith Tanner MegaDork
7/14/17 9:44 a.m.

I'd prefer to avoid switches. I want this thing just to take care of itself

I wired the whole system myself, so there are no surprises. I don't really care about using the house batteries as a starter, if it comes to that I'll just start moving batteries around. If nobody sees a problem with the isolator concept, I'll give that a shot.

oldopelguy UltraDork
7/14/17 10:16 a.m.

Does the solar setup actually have a voltage regulator or is it just one of the ones designed to hook up to a car battery and charge it? If it is the first type, then carry on.

If it is the second type, do you care if you lose a little charging on the house batteries to keep the starting battery up? Assuming you don't, since that is why you asked, what is the max output of the solar system in amps?

Assuming less than 5A or so, the easiest and cheapest answer is a pair of diodes, one from the solar panel to each of the batteries. That is essentially what those battery isolators are, but cheaper. In fact the cheapest option is to get a "full wave bridge rectifier", which is essentially an array of four diodes in a single component. With a 25A FWBR available for @$4, you could be done for coffee $. Hook the positive from the solar to the dc terminal marked (-) and the two batteries to each of the (AC) terminals.

Two batteries hooked up off a pair of diodes forms what is known as an auctioneering circuit. Essentially the battery with the lower voltage is going to get most or all the charging until it's voltage raises to higher than the other battery, at which point the charging will shift. Essentially the battery in worse shape is going to get the charge. In my own setup that would be what I want, though I would probably add in some manual switches that would let me manually configure differently if needed.

java230 SuperDork
7/14/17 10:27 a.m.

Why not run an isolator between the house and start batteries, then connect the solar to the start batteries?

When solar is making power, isolator closes connecting house and start batteries, charging both, when input from panels drops isolator disconnects. Isolator will still connect when alternator is charging as well.

Keith Tanner
Keith Tanner MegaDork
7/14/17 10:31 a.m.

Solar setup runs an MPPT controller. It's a 100W panel, which is around 6A.

I know what a bridge rectifier is although I associate it more with converting AC to DC. Here's the isolator I've been looking at - it uses FETs instead of diodes and offers less voltage drop. I don't care about voltage drop when running off the alternator, but solar power is precious and inefficiency bothers me intellectually. I get upset if the house batteries aren't full by noon

I said earlier I didn't want switches, but that might have been hasty. I'd probably want the ability to decouple the starter and house batteries while camping so I can take full advantage of the solar if I'm running a high draw or have poor solar coverage. So basically a switch to go from "storage mode" to "camp mode". In that case, some big inefficient diodes would probably do the trick. I need to sit down and sketch this out.

wlkelley3 UltraDork
7/14/17 11:23 a.m.

I would probably go with a separate smaller solar system designed for starter maintenance. The existing stuff works great, leave it alone.

Dr. Hess
Dr. Hess MegaDork
7/14/17 12:29 p.m.

Yeah, I was gonna suggest a separate solar cell set that is designed to just do car batteries. I have one on my RN Truck (Harbor Freight), one on me RAV (MIL Spec) and an extra HF one laying around that I intend on putting on my generator when I get to it. They are cheap at HF. Don't over think it.

Or, go nuts with diodes and current limiter circuits and link the 2 battery sets together. I prefer simple and cheap.

bearmtnmartin SuperDork
7/14/17 12:44 p.m.

You need one of these. It keeps both sets of batteries charged and linked, but when the voltage drops below 12.5 or so, it will delink the start battery to prevent it from discharging. I have this on our bus and it eliminated my worries about not being able to fire up the bus on a Sunday afternoon. A nice bonus is that you can pull a little charge from your start battery to add to the house bank when it is fully charged, which gives you a few more amp hours.


Keith Tanner
Keith Tanner MegaDork
7/14/17 1:58 p.m.

I prefer effective over cheap. I'm not very GRM that way

I'm gonna doodle it out and come up with all my use cases. I don't want to just gang the two banks together, because they will not get along unless there are diodes in there. I may be able to use an isolator to do a better job of hooking up the battery banks when under way as well.

APEowner Reader
7/14/17 2:57 p.m.

This is horribly inefficient but the setup I use in the race trailer might work for you. I've got solar panels that charge the house batteries(well, one big, stupid expensive, battery)which run the lights, burglar alarm, the trailer brakes if the breakaway switch is pulled and an inverter that powers the laptop. When the race car is in the trailer I use a battery tender plugged into the inverter to keep the race car battery charged.

Dr. Hess
Dr. Hess MegaDork
7/14/17 3:34 p.m.

You're making this way too hard, Keith. You put this on your dashboard, plug it into a always live socket or wire it in to an always on circuit and you're done. Simple, effective, cheap. But, I'm sure you can rig up something that costs 10x if you really try. Go for it.

Rodan Reader
7/14/17 3:37 p.m.

I don't have anything useful to add at this point, but I'm going to listen in because I'm installing a deep cycle battery and winch in the race trailer next week, and a solar charging setup is in the near future plans...

Keith Tanner
Keith Tanner MegaDork
7/14/17 5:43 p.m.
Dr. Hess wrote: You're making this way too hard, Keith. You put this on your dashboard, plug it into a always live socket or wire it in to an always on circuit and you're done. Simple, effective, cheap. But, I'm sure you can rig up something that costs 10x if you really try. Go for it.

I take pleasure in clever, thoughtful solutions. More so than throwing a piece of cheap Chinese crap made to a minimum price on the dash and crossing my fingers that it won't set itself on fire. I've been inside enough super-cheap electronics to not trust them with anything I care about.

When I'm done, I'm sure I'll have spent more than $13.99. I'm OK with that. The vehicle will be more capable and better engineered than it was when I began, able to take care of itself. And I'll have learned something. I place value on all those attributes. Could possibly be why I'm decently effective at my job, when I think about it.

If I just wanted super-easy and cheap, I'd run an extension cord out to the van and plug it into one of my battery chargers.

codrus UltraDork
7/15/17 12:08 a.m.

I think what you want is this, Keith.

It's not cheap (runs about $100), but it's designed to do exactly what you want.

I put one in my trailer to charge up a pair of group 31 batteries using the 12v feed from the truck.

Keith Tanner
Keith Tanner MegaDork
7/15/17 11:34 a.m.

If it saves one battery, it's saved me money. I'll have a look at it, thanks.

qdseeker Reader
7/17/17 8:01 a.m.

I only have the isolator in my engine compartment for my set up. It's set like yours in that when the engine is off, the camping battery is charged via the sun. When I start the bus, the battery automatically starts being charged by the alternator. My bus literally sits for 5 days straight because I drive my work vehicle home and I have no issues firing up each time. Been running it for 2 years now.

Keith Tanner
Keith Tanner MegaDork
7/17/17 11:27 a.m.

On the recommendation of another Westy guy, I've ordered one of these.

Looks perfect. And it's only four times the price of a HF solar charger

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