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Solar Question for you Tech Minded | Category: Help

itsaboat 4 years ago

There are bound to be people here running solar power on their boats, caravans etc. Maybe someone can answer this for me.

I want to run a DC-DC 20 amp charger from my caravan (house battery) to charge a second battery. Is there any tech reason why this cannot be done?

The details...The "experts" told me, or rather insisted and in some cases DEMANDED that what I have done cannot be done. So I done it and proved them all wrong.

I have a 120 watt bi-fold 12 volt panel, which I re-wired in series to make it a 120 watt 24 volt panel. It is mono. It then runs through a 12/24 volt 20amp auto switching MPPT controller and charges the 120AH van battery...That is the part the "experts" say cannot be done !! I have been running this set up now for 2 years, so I have proven it can be done. What this achieves is awesome charging ability. I run an Engel on freeze mode, some LED lights, charge the laptop, watch some TV, etc and use about 50 to 60 AH per 24 hours. On most days the house battery reaches float by 10 in the morning (the experts say that cannot be done too) and on a good day by 9 AM. Even with the Engel running on freeze, the battery stays at float all day until the sun goes down.

I am clearly producing a lot more power than what I am using. So I thought I might run that excess power into a second battery which I can then use to run the inverter. The biggest thing I would run off the inverter is my little washing machine which is 230 watts. So about 23 to 25 amp draw for about 12 minutes. It's a 1000 watt (2000 surge) sine wave inverter (as apposed to a modified square wave !!).

Both batteries are wet cell LA. The house battery is sealed valve regulated calcium wet cell, 120AH. The second battery is open valved lead wet cell, 100AH.

The instructions say the DC-DC is intended to be used as a dual battery charger. But they make no mention of solar. I don't want to burn out my brand new DC-DC charger the first time I plug it in

Thanks in advance for any advice you might have.


itsaboat mate...Life is just a boat and then ya marry one !
itsaboat 4 years ago

An update.

I have received a reply from an "expert". He did not actually answer my question, so he gets no promo link to his web site !!!

Can I charge a second (stand alone) battery off a primary solar charged battery via a DC-DC charger? Simple question.

He suggested that connecting the two batteries in parallel and charging both off the MPPT would be better. He also suggested that using the DC-DC charger would causes loses.

OK. Good. I appreciate his reply. But I already understand what he has said. What he has suggested is what would normally be done. So I figure I will just be going around in circles again with "experts" telling me it cannot be done...So I am just going to do it.

If the only issue is loses, then my new DC-DC charger should be OK.

The forecast rain has arrived. So next week when the rain clears and the sun comes out, I will give this a test run. I want to push it hard so that if anything is going to break it does ! and I need plenty of sun to push it to the limits and try and break it

BTW. What the "expert" (although I do appreciate his input) has suggested is not as simple as just connecting the two batteries together...Relays, BMS, voltage regulator etc, etc all adds up to 2 or 3 times the cost of a DC-DC charger...AND the DC-DC charger has all those things built in...And if you did want to parallel two batteries like that, where would you purchase all those bits and pieces from? - From an "expert" of course !!

itsaboat mate...Life is just a boat and then ya marry one !
itsaboat 4 years ago

I have had the setup running now for three days. It is working !!

So I now have power capacity to last 2 1/2 days with out any form of charging (that is 24 hour days).

The only issue I have found so far, is the solar panel cannot keep up with the DC-DC charger. However, the DC-DC has all the normal safety features, over charge, discharge, surge etc, etc protection built in. It stops charging when the main battery falls to 12.5 volts which then gives the solar a chance to catch up. So it is not a problem and in fact it is exactly how it is supposed to work. But it is an issue to bare in mind especially on those heavy usage days, but it's nothing more than normal solar awareness.

So i can now go camping and fishing for 2 1/2 days in total darkness and still have a freezer, laptop, phone, watch TV and even do a load of washing...and I really don't think 60 straight hours of total darkness is going to happen !!

itsaboat mate...Life is just a boat and then ya marry one !