Trouble wrapping my mind around batteries/solar/etc

Hey ya’ll, I’ve been reading a ton of the posts on this forum and love the variety of tips between people like Bretly, Van Dweller and others on power consumption vs intense battery/solar set ups. I was wondering if I could ask for some advice on deciding what to do for power in my sprinter?

I’ve tried reading a few articles about the different options and sifted through some of the power use charts and I’m just struggling to wrap my head around it all. When we bought our sprinter (2006 Dodge 3500) it already had two secondary batteries, a power inverter and what I believe is an isolator. Right now we’re able to run a couple of LED lights and a roof vent fan for a few hours before the inverter beeps to let us know the batteries are running low. The only charge the batteries are getting is from driving around.

Ideally I want to figure out how to have enough power to run the lights and fan for much longer each day and have enough power left over to charge two lap tops and two cell phones. Do you all think I can get away with avoiding solar (maybe by upgrading my batteries?) or should I bite the bullet and invest in a simple solar kit? I wont be living in the van full time, it’s more for longer road trips, so I’m hesitant to put tons of money into a complex set up.

Here’s the set up as it stands:

Are you running everything using alternating current through the inverter? I assume you are since you say the inverter is beeping.

Inverters are not very efficient. It’s far more efficient to run only 12 volt lights, fans, etc. There are plenty of 12 volt lights, fans, and other appliances on the market - no need to even turn the inverter on. Leave it off unless you have to use it - even without a load it consumes a lot of power.

What condition are your batteries in? Have you kept up with topping off the electrolyte level in the batteries with distilled water? If you haven’t, they might be ruined.

Also, it seems that your batteries aren’t pure deep cell types. If it’s a starting battery or hybrid starting/deep cell battery it won’t last as long as a true deep cell battery (like golf cart batteries). A clue that it’s probably not optimum is when they list cold cranking amps on the top of the battery - it’s a starting/hybrid battery that isn’t ideal as a house battery.

Once you get your batteries figured out, do the math on how much power you use every day vs. how much you charge the batteries every day. If you’re not giving them more charge than you’re using, then you’ll eventually kill the batteries. For example, if you use 30 amp-hours every day, but only giving 25 amp-hours of charge every day, you’re losing 5 amp hours a day (more actually, because charging takes more energy than what the batteries provide), then you’re running a deficit and you’ll eventually kill the batteries. If you over-discharge lead-acid batteries they’ll lose capacity and eventually fail.

Gotta do the math.

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If, after you calculate how much you’re charging vs. discharging and you’re running a deficit, then you might have to supplement the charging routine with something like solar. You might not need solar if you drive every day or two and your power needs are low.

No free lunch. Once solar is installed is kind of a free lunch as long as you properly manage your power needs.

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Hey Axel, thanks for the reply! The lights I have are currently running off the inverter but the ceiling vent/fan is 12v. I’ve only had the van for a couple of months and haven’t used it much yet, so I’m not sure what condition the batteries are in - is there a good way to check? I wasn’t aware that I have to add distilled water, I’ll look into that now. Should I look into replacing them with batteries that are newer and more optimal?

For the math on my power usage, taking my lap top for example, I need to figure out how much power it uses, then estimate how many hours a day I’ll be using it? Then do that for all the devises I plan on using daily? Sorry for all the questions, I appreciate you chiming in.

You can bring the batteries to an auto parts store and they might be able to do a load test. But first I’d lift the caps to see where the electrolyte level is. The two house batteries together in the picture (ones that say “Marine”) are wet cells, but the starting battery might not be - don’t try to pry the covers off a sealed battery.

Be careful - the electrolyte is sulfuric acid. Wear gloves and eye protection and have water handy to rinse it off your skin or flush your eyes. There will be three openings under each plastic cap, two caps per battery. After cleaning the top of the battery around the caps, slowly pry the caps off with a screwdriver. Inside you’ll see a little black plastic tab molded into the battery hanging down inside each cell. The electrolyte level should be as high as that tab. If the tops of the lead plates are visible above the electrolyte it might already be too late, but you could still try to save them by adding distilled water and charging them. Use only distilled or de-ionized water. If you use tap water it may ruin the battery due to the minerals in tap water contaminating the battery.

You may not have to replace the batteries if they still meet your power needs, but you’ll have to figure out your power needs.

You’re on the right track - you’ll need to add up all your appliances and multiply by the number of hours they’re used, then compare that to your charging situation.

Some of this post explains how to calculate your needs:


Curious as to what your inverter deems as low voltage? If it’s below 11.8 - 12v you might be damaging the batteries or may have if you continued to use them afterwards. If it’s something like 12.2v you may not be using all your available power. Just something to consider looking at. The voltages differ for battery types and in my experience low voltage warnings are different on different devices as well.

I would also meter the voltage at the batteries and compare it with the inverter to make sure there is no voltage drop there. We had a cheap fridge at first that alway showed the voltage 0.3 lower than what we read at our batteries. The wiring was fine and as best I could tell it was something inside the fridge or the meter in the fridge itself that was just wrong. Definitely want to rule out the false negatives as well.

Good luck!

Good point on the voltage drop. Be sure you’re using the correct size wire for the load and your connections are clean and tight, otherwise the voltage might drop a lot.

I brought all the batteries to an Auto Zone and they charged them up and said that they were in good shape. So that’s good. Should I still pry the top off and take a look?

I also tried to estimate my power needs and came up with this. Does it look like I’m doing this correctly?

That’s a good question - I don’t have a voltage meter but it sounds like that would help with all this

Do you know how I can figure out how much power the isolator is funneling into the house batteries while I’m driving? Also, so I don’t feel like a total idiot, does that third picture in my original post look like an isolator?

The third picture does look like a relay/solenoid type isolator.

You’re using a little more than 100 amps a day to start with, and most is being run through an inverter. There’s a lot of inefficiency associated with converting DC to AC, so you’re probably more in the neighborhood of 120 to 150 amps per day of usage. That’s a lot. You can probably save quite a bit by ditching the inverter and going to all DC chargers and appliances.

You’ve figured out your usage budget, now you need to figure out the other half - the charging side of the budget.

To figure out how much charging you have, you have to figure out how much the alternator puts out vs. how much it takes to run the van’s essential needs such as the ignition system, lights, sensors, blowers for the AC and heating, ECU, controls, etc. What you have left over is what’s available to charge the house batteries. It might not be much. It might only take a half hour of driving a day to meet the house battery charging needs, or it might take 5 hours a day. You’ll have to make up the difference somewhere, either by installing solar, plugging a battery charger into shore power, running a generator etc.

It’s much easier to reduce your power needs than to increase power generation. I would start with converting everything to DC and ditching or at least minimizing inverter usage.

And yes, flooded lead acid batteries have to be maintained by topping off the cells with distilled water. I would pop those caps off every month to top them off.

If you decide to install solar, in your situation I would install a 300 watt solar panel. Basically the same type of panel that most people are putting on their homes these days. Since home solar panels put out more voltage than smaller cells, use an MPPT controller.

The 300 watt panel combined with occasional driving should be more than enough to meet your needs. Your needs will be mostly addressed by solar.

Here’s a tutorial I wrote on solar install. If you know the very basics of DC power you should be able to do it by following it.



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