diy solar

diy solar

Can I use a battery as an input source for a mppt charge controller

seangirard

seangirard

New Member
Joined
Feb 1, 2022
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5
Ok, this might be a dumb question. I have a setup that I've been running for a couple of years and it works great. Battleborn battery wired as the "coach" battery in my van with a Victron 20amp MPPT. I have a port on the side of my van where I can connect foldable panels when I'm camping. It's great.

Sometimes I'm heading out in conditions where I know there won't be much sun. There is no space available to permanently wire in another battery, and a lot of times I don't need any extra capacity, but now that there are some less expensive LiFePO4 available, I'm interested in possibly bringing one along sometimes as a backup source.

I know there would be some efficiency loss, but I'm thinking the most convenient solution would be if I could use Anderson connectors or something alligator clipped to the "external" battery and just connect it to the input of my MPPT charge controller. Is something like this possible? Would I need to regulate the voltage going into the MPPT? Is there a better way to achieve something similar?

Thanks!
 
Yeah, you could just install a Blue Sea Systems A, B, Both A+B selector switch somewhere handy inside, and run the Anderson connector to a good spot outside, where you can sit another battery on the ground under the van a bit sheltered, and then plug it in via Anderson like you suggested, but I'm saying not to use MPPT, but rather Blue Sea Systems switch to parallel them, or have the choice on which battery to select separately or joined.

Ref:

Manual Battery Switches - Blue Sea Systems

Engineering high quality marine electrical components for safety, reliability and performance
www.bluesea.com www.bluesea.com

Just to note though. You want to make sure you have a disconnect on the MPPT circuit as well, and turn off the MPPT input prior to switching batteries around on the bank, because some controllers can be damaged if battery bank goes missing while MPPT is getting juice from the panels.

If you left the selector in such a way that one battery is always hooked up it would be fine, but if you ever went to Off position or in between positions too long, it could damage the charge controller if the solar is alive. Like if it was normally set to A, and you hooked your battery in and set it to A+B it would probably be fine, or if you went from A to B quickly, it probably wouldn't be harmful, but you wouldn't want to break the battery connection for more than a second or two unless you disconnect the solar PV input prior.

And of course you want to make sure the Anderson connector is properly rated for the amperage required to feed your inverter or other loads running through it.

One other option you can explore as well, is a DC-DC charger like they use on RVs. They are not as high of amps, but it would charge the one battery from the other, like 15a.

Example:
 
Thanks all for the responses. I am planning to add a dc-dc between the starting battery and house battery. Maybe I need to think about hooking into that.
 
seangirard said:
Thanks all for the responses. I am planning to add a dc-dc between the starting battery and house battery. Maybe I need to think about hooking into that.
Click to expand...

Renogy also makes the DC-DC charger (for main battery to house battery charging), that also has an MPPT input on it for solar input:
www.renogy.com

DCC50S 12V 50A DC-DC On-Board Battery Charger with MPPT

Make sure your service battery has all the juice it needs with Renogy's DCC50S DC-DC MPPT On-Board Battery Charger. 10% off: Renogy10off. Free shipping
www.renogy.com www.renogy.com
 
Samsonite801 said:
Renogy also makes the DC-DC charger (for main battery to house battery charging), that also has an MPPT input on it for solar input:
www.renogy.com

DCC50S 12V 50A DC-DC On-Board Battery Charger with MPPT

Make sure your service battery has all the juice it needs with Renogy's DCC50S DC-DC MPPT On-Board Battery Charger. 10% off: Renogy10off. Free shipping
www.renogy.com www.renogy.com
Click to expand...

Not a bad unit, but it has some limitations that have bit people in the butt:

50A max solar OR 50A max DC-DC. If both are present, both are limited to 25A EA
25Voc max PV input on the MPPT - very limiting - basically only allows 12V panels in parallel.
 
sunshine_eggo said:
Not a bad unit, but it has some limitations that have bit people in the butt:

50A max solar OR 50A max DC-DC. If both are present, both are limited to 25A EA
25Voc max PV input on the MPPT - very limiting - basically only allows 12V panels in parallel.
Click to expand...

Yeah, I've never looked into it that close, just seen some others using them before. Thanks for pointing out those limitations...

He already has an MPPT charger anyways, so probably he could just add a Victron DC-DC charger or something...
 
I tried that with an enphase M215 and it let the smoke out. I assume the MPPT was expecting a current limited source (solar) but when I plugged in the 24v battery the current was essentially unlimited and blew something. Once I limited the current inrush then the mppt would accept the battery as input and run the inverter.
rearden
 
rearden said:
I tried that with an enphase M215 and it let the smoke out. I assume the MPPT was expecting a current limited source (solar) but when I plugged in the 24v battery the current was essentially unlimited and blew something. Once I limited the current inrush then the mppt would accept the battery as input and run the inverter.
rearden
Click to expand...
How did you limit the current from the battery to the MPPT?

I'm planning a 72v bank on a sailboat for a propulsion motor and a separate 12v bank for house loads. I'd like to have the solar panels charge the 72v bank and the 72v bank charge the 12v bank. I thought of having the 72v bank feed an MPPT for the 12v bank, but only because I could not think of something else.

Any input would be appreciated.
 
ERNWA said:
How did you limit the current from the battery to the MPPT?

I'm planning a 72v bank on a sailboat for a propulsion motor and a separate 12v bank for house loads. I'd like to have the solar panels charge the 72v bank and the 72v bank charge the 12v bank. I thought of having the 72v bank feed an MPPT for the 12v bank, but only because I could not think of something else.

Any input would be appreciated.
Click to expand...

That should work. I would wire and fuse it for the current anticipated, e.g.:

you want to charge the 12V @ 30A max:

14.8V * 30A = 444W

444W/72V = 6.2A

Use wire rated for 10A and put a 10A fuse in the line.
 
sunshine_eggo said:
That should work. I would wire and fuse it for the current anticipated, e.g.:

you want to charge the 12V @ 30A max:

14.8V * 30A = 444W

444W/72V = 6.2A

Use wire rated for 10A and put a 10A fuse in the line.
Click to expand...
Thanks! Just wanted to be sure it was nothing too complicated.
 
A MPPT controller does have a DC-DC converter as part of the circuit. But a DC-DC converter cannot optimize the input voltage to achieve the maximum power point from the source.

Common to both MPPT controller and DC-DC converters is the need to disconnect the input source before disconnecting the load output. Not removing the input before disconnecting the output results excess voltage being generated which can result in the output switching devices being forced into higher voltages than designed for with the result that the devices are exposed to voltages that the unit is designed for. This can result in component failure.

ALLWAYS......remove input voltages first, wait for the capacitors to discharge before disconnecting output load.

SPECIAL.....instructions for Midnight MPPT controllers,....turn off input breakers.....press the center button, (force sweep) , this forces discharge of input caps, then turn off output breaker. This procedure may be valid for Outback as well, but not sure. Morningstar does not have “forced sweep”
 
The Outback Flexmax 60/80 will operate down to 2V over battery voltage, which is still a bit shy for a same voltage battery to charge it. They also have a mode where you can select a percentage of Voc detected and the controller will not seek outside your setpoint.

This is the only info I can find on power off/rebooting. I don't see anything about power off sequence in the manual.

Outback FM60/80 manual said:
To reboot the FLEXmax:
1. Turn off the battery and PV array circuit breakers. The FLEXmax should have no sources of power connected. Observe the screen until it goes blank.
Click to expand...
 
Samsonite801 said:
Yeah, you could just install a Blue Sea Systems A, B, Both A+B selector switch somewhere handy inside, and run the Anderson connector to a good spot outside, where you can sit another battery on the ground under the van a bit sheltered, and then plug it in via Anderson like you suggested, but I'm saying not to use MPPT, but rather Blue Sea Systems switch to parallel them, or have the choice on which battery to select separately or joined.

Ref:

Manual Battery Switches - Blue Sea Systems

Engineering high quality marine electrical components for safety, reliability and performance
www.bluesea.com www.bluesea.com

Just to note though. You want to make sure you have a disconnect on the MPPT circuit as well, and turn off the MPPT input prior to switching batteries around on the bank, because some controllers can be damaged if battery bank goes missing while MPPT is getting juice from the panels.

If you left the selector in such a way that one battery is always hooked up it would be fine, but if you ever went to Off position or in between positions too long, it could damage the charge controller if the solar is alive. Like if it was normally set to A, and you hooked your battery in and set it to A+B it would probably be fine, or if you went from A to B quickly, it probably wouldn't be harmful, but you wouldn't want to break the battery connection for more than a second or two unless you disconnect the solar PV input prior.

And of course you want to make sure the Anderson connector is properly rated for the amperage required to feed your inverter or other loads running through it.

One other option you can explore as well, is a DC-DC charger like they use on RVs. They are not as high of amps, but it would charge the one battery from the other, like 15a.

Example:
Click to expand...
You cannot use a manual switch to connect LFP or other Li-ion batteries to each other or to lead-acid batteries if their voltage differs by more than around 0.1V as the internal resistance is very low and can cause overcurrent.
 
Alan Cave said:
You cannot use a manual switch to connect LFP or other Li-ion batteries to each other or to lead-acid batteries if their voltage differs by more than around 0.1V as the internal resistance is very low and can cause overcurrent.
Click to expand...

Of course you need to make sure you have proper overcurrent protection (to protect your cables).

I've had my two battery banks (LFP and SLA) bridge-connected for 2 years now and no ill side effects (I can turn on or off the bridge switch at will), it's a great way to keep all those old SLA batteries from just going dead sitting there doing nothing. I don't really care about them much anymore but if I can keep them for a few more years I might as well.

I also use that SLA bank to plug in dead batteries onto temporarily (let them sit there a few days) and it brings them back, like dead batteries from tractors or field equipment, I just throw on my jumper cables and leave them on for a few days until I can see whether they come back or if they have a dead cell or something else.. Most wind up good, then I can take them off the jumper cables and go put them back into the equipment again.
 
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