HI All,
Total newbie here - I designed a simple grid battery backup system for my house. That is the only purpose of this system - to provide backup power when grid is down. It consists of:
160 watt solar panel
VC SmartSolar MPPT 100/20 solar charger
Renology 2000 watt inverter with ATS and bluetooth
four 100AH 12 volt deep cycle lead acid batteries in parallel (from golf cart that got Lithium upgrade)
All works well, except that when solar charging fades (e.g. overcast days, nighttime), the inverter slowly drains my batteries with about a 1.5 amp draw even when connected to grid power. The issue is that this drain can leave my batteries with less than a full charge - which impacts how long my battery backup system can supply power in the event of a grid outage…
So, I am wondering what is the easiest way to get around this issue?
Attaching 12V trickle charger (e.g. max 2 amps)? Could I connect trickle charger directly to batteries - or would it not peacefully coexist with the solar charger?
Attach a DC power source to the solar panel connection at solar charger? My panel is rated 18V max I believe, so maybe a DC power source close to 18 volts and capable of a couple amps?
The inverter likely has an idle power consumption around 18W.
Adding an AC charger would allow for float charging the battery to keep it at maximum capacity.
The charger will impact renewable energy harvest however, as the full battery won’t have anywhere to put the additional energy so it will go uncollected.
The overnight drain will be on the order of 0.25kWh. If that’s compromising backup system runtime enough to cause concern, then perhaps the battery is too small for the use case.
Thanks, but not sure I fully understand what you’re saying…
Yes, 18w is the power consumption of the inverter on the 12 volt batteries. These four 100ah deep cycle lead acid batteries are older (came out of my golf cart). Just trying to reuse them, rather than discard. But even if I had new batteries, the constant 18w drain on the batteries during no/low solar charging is not ideal for my battery backup scenario. I want those batteries to be fully charged every time there’s a grid outage…
When you said “AC charger” - I assume you mean some sort of trickle charger (plugged into grid AC) that is connected to the bank of parallel batteries? And isn’t there some type of charger that would throttle down its output based on battery voltage? If so, that would be the ideal scenario - I think…
Forget choice 2, because when the MPPT is doing its scan will short circuit the DC power source.
Choice 1 is the way to go.
Any charger will throttle down its output when the set voltage is hit.
Will work as a constant current (CC) source during charging and then, on the final stage, as a constant voltage (CV) source.
Thanks Alex! Glad to hear my thinking was correct about most trickle chargers throttling down when max target voltage is reached at batteries. But how do things play out when there are multiple charging sources operating concurrently? e.g. the solar charger is also attached to the bank of batteries…
Any type of 12V trickle charger rated 12V @ 2A would ne enough to make up the inverter’s idle current consumption. It will help keep the battery topped off if fully charged by the MPPT charger initially.
A multi-stage “smart” charger will charge the battery from any state of charge whenever grid power is available. A 40A charger would be 10% charge current which is a typical target for a 400Ah lead acid battery.
The AC and MPPT chargers can work in parallel to charge the battery. I’ve connected dozens of AC chargers to renewable energy systems without the need of a blocking diode.
Just bought this Victron trickle charger. I know I could have bought a generic charger for less than $40, but I like the VC charger features, including monitoring it with the same VC app as used for my VC solar charger…
Just noticed it’s rated 5A - so a bit more than a trickle charger, but will come in handy to more quickly charge depleted batteries after a long grid outage…
