I am modifying my cargo trailer to be more like a camper, and dug up some old stuff for it.I have a modern MPPT solar controller for the 100 watt (soon to be 200w).Two golf cart batteries.I have an old 12.7 volt, 15 amp DC power source of a 1972 Winnabago, attached to a 115 circuit breaker box complete with shoreline/generator switch.I am sure there is no monitor on the DC charger/power source.The solar I know how to set up no problem, no questions.The power source no problems either.What I want to do is have the shoreline DC power source be "smart". I do not want to just send 15 amps to the battery everytime I plug in or want to use the DC power source instead of the batteries. I will be running a Danfoss chest fridge, and just minimal lights and minimal charging, min fan etc.I do not want to buy a $500 controller that controls both power sources (solar and charger) electronically, but can spend under $50 for something to make it all work together.The breaker box does have a breaker for the charger if it comes to that so I can still plug in and have shore power for AC or electric heat without charging (overcharging) the battery.I would rather have something monitor the battery automatically, so looking for a thing maybe to go between charger and the battery to control the charge while the solar works in the daytime but not at night.Keeping the battery in float while running the fridge and outlets off the charger/converter is the most preferred solution. Anyone tackled this?

12.7 volts is not enough to charge a 12v battery. That power source would just be able to run some 12v things such as lights and fans.It is useless for what you want to do.BTW it does not "send 15 amps". What it does is provide that 12.7 volts. Whatever amps you get depends on how much lower the "load" is in voltage. Where the battery is the load at say 12.4 volts, the 0.3 volt difference will not make for many amps at all.You just have to triple that $50 budget and get a proper converter. EG:https://www.boatandrvaccessories.com/products/powermax-pm3-60lk-12-volts-60-amp-power-converter-battery-charger-w-led-light

“I do not want to buy a $500 controller that controls both power sources (solar and charger) electronically, but can spend under $50 for something to make it all work together”Good MPPT solar controllers are way under $500. Try $199. https://www.windynation.com/Charge-Controllers/Windy-Nation/WindyNation-TrakMax-MPPT-40A-Solar-Charge-Controller-12-or-24-Volts-for-Sealed-AGM-Gel-and-Lithium-Batteries-with-Opt/-/3403?p=YzE9MTc=Did you mean a controller that handles solar and 110?

hmm, Im not sure I follow your logic on amps.You are right, if it is only 12.7 volts then that will not matter if the solar is charging at 14.This is 1970's technology, 15 amps is a decent amount of power to flow continuously. I don't think the 15 amps are variable, I think it just puts out 12.7 at 15amps, so the battery state will not alter what the charger does, it just does it no matter what. How the battery takes the power is different.Am I missing something?I will have to take actual measurements when I can plug it in and connect to a battery. I cut this out of my wrecked Rv a few years ago, but remember when I switched to the charger (not battery) that the lights got much brighter, so that suggest more like 14 volts than just 12.7 as batter would be12.4-12.6 at the same time…The solar and MPPT are all that is needed to take care of the power, but on rainy days, or heavy shade, or cloudy short winter days I would like the option to supplement…

It is not possible to construct a charger that simultaneously provides a fixed constant voltage and a fixed constant current. The two are intimately related; at a given state of charge (and temperature and so forth), there is a single voltage vs current curve that applies to the battery. It is closely related to Ohm's law, although the relation between voltage and current is not precisely linear in this case.Your charger might put out 13.something volts under a small load (low current required) which tapers down to 12.7 at 15A. There are plenty of other things that could be going on, too, such as seeing the effects of an old split output converter.At any rate, I think you would be well served to replace the charger with a modern multi-stage unit, such as a Progressive Dynamics PD9245. It is more than $50, but worth the money and kinder to your batteries. (I guess if you rely on solar nearly exclusively it makes little matter what converter or charger you have.)

Hi,I'd want to "see" the output on that old power supply. I'll bet you dollars to donuts it has lots of "ripple". I would certain NOT use it.

Solar and Charger into one? | Good Sam Community

BFL13
Explorer II
Jan 26, 2020
All charging goes to the loads first, then the batteries get anything left over. If the loads take it all and want even more, that has to now come from the battery.

That is why RV converters are set to 13.6v as "normal" for while camping, and only go to 13.2v "storage" for battery maintenance when not camping and there are no other 12v loads.

The OP's 1972 converter needs to be checked for actual voltage no load and then when under the expected 12v loads, to see if it can hold the loaded voltage up enough so the battery does not have any draw on it.

The loads should be connected on the other side of the battery from that old converter so the battery will act as a filter for any "dirty 12v" that could ruin the fridge controls and any other circuit boards in the various 12v loads.

BTW, did we discover whether that "15 amps" is its 120v rating for the AC breaker panel or indeed for its DC output rating?
time2roll
Nomad
Jan 26, 2020
Yes the battery voltage will sag if the load exceeds the solar charging capacity. Then voltage will recover when the load is reduced and the solar gets back to charging. A good converter will hold the battery very steady at about 13.6 volts. That old converter might help you out if plugged in but if you expect a fast charge on generator power you will need a new modern multistage converter.
BFL13
Explorer II
Jan 26, 2020
If you have been running the load for a time, the voltage will only bounce back to 12.4, not 12.5. Amp Hours. Don't forget the hours.
red31
Explorer
Jan 26, 2020
A load tester apples a large load to a battery and hopefully the voltage does not drop below 9v. Turn it off and the voltage returns to near where it started.
ajriding
Explorer II
Jan 26, 2020
That's true.

When I use power off the battery I think the voltage drops, so then the charger will be able to supply current before the actual battery voltage is low.
Is this right? A 12,5v battery might read 12.0v when you are using power, but stop and it goes back to 12.5v, so there is a false low voltage reading, or an instantaneous charge read across the terminals…
A single LED bulb will not do much, but the blower on the heater will.

I may pull this converter off and use it. It is al attached on a board with the breaker box and one outlet and a ground bar, so is big and heavy.

I did not wire the batt into the alternator as I never needed that on previous camper, but do want a way to charge in emergency
time2roll
Nomad
Jan 25, 2020
The solar controller and converter feeding a single battery is not an issue ever.
If you need a power center with quality converter consider the PD4045
http://www.bestconverter.com/PD4045-45-Amp-Inteli-Power-Mighty-Mini-Power-Center

Any decent stand alone converter is only a few $$ less.
full_mosey
Explorer
Jan 25, 2020
ajriding wrote:

Anyone tackled this?

Yep!
See my profile.

HTH;
John
BFL13
Explorer II
Jan 25, 2020
Using a charger instead of a converter means using the batteries until they get low and then recharging, repeat.

A converter will stay on at 13.6v and run your 12v loads with no draw on the batteries, so that is what you want. It can also act like a charger when required to do so.

A "trickle charger" that stays on constant voltage at low amps would be ok for storage time battery maintenance, but while camping the low amps means it likely can't run the 12v things in the rig all by itself, so the batteries will still get low.

Therefore you want a converter with enough amps. AFAIK, this is the lowest priced one that can do the job you want as long as your total expected 12v draw at any time for a mix of 12v things is under 15 amps. $87 and free shipping:

https://www.boatandrvaccessories.com/products/powermax-pm4-15-converter
pianotuna
Nomad III
Jan 25, 2020
ajriding,

The acceptance rate of charge on a lead acid battery drops to about 12.5 amps per 100 amp-hours of capacity at 85% of full. As the battery becomes more and more charged that number drops a heck of a lot. Increasing the charge voltage simply causes the battery to gas. It is possible to fully charge at 12.8 volts--but it would take about 176 hours.
ajriding
Explorer II
Jan 25, 2020
thanks.
Maybe i will just keep the old charger/breaker box in storage…It is heavy at 20-25 lbs anyway.

I realized too that the chest fridge will prefer AC power when available, duh, so there will not be a lot of demand on the battery when plugged in.
Maybe I can just have a trickle charger available.

I am doing a lot of unconventional things with this project, so while a fun challenge, is complicating it all.

I am thinking of mounting the two batteries on the outside in front of the jeep fenders (aerodynamically neutral) and also the generator behind the fender. Literally, thinking outside the (cargo) box..

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