So I have a 75|15 charge controller which currently charges my LifePO4 house battery from solar. But my AGM starter batteries have no charging when I’m off grid, except from alternator which is used sporadically, yet the AGMs get used daily for anchor lighting. So sometimes the AGMs take a beating and get run down below 50% which I know is not good. And my LifePO4 only gets charged from solar, but not at all from the alternator or shore power. The normal (but expensive!) solution is a B2B charger from AGM to LifePO4, plus a trickle charger for the AGMs from the LifePO4. So I’m trying to squeeze a little more out of the system I have for a while.
And I’m wondering if switching the MPPT to the AGMs and then using the Load output of the MPPT to charge the LifePO4 when the AGMs are full is maybe a better choice. The AGMs will be topped up daily from solar, then the remainder of the daily solar charging goes to the LifePO4. And when the alternator is running or when on shore power, then the LifePO4 gets charged as well as the AGMs. I figure I could set the voltage parameters to charge the LifePO4 when the AGMs are over 13V, and stop charging when the LifePO4 hits 14.2V or so, which is basically full or close to it.
I used to worry more about charging the LifePO4 correctly. But considering the AGMs are more expensive, and more ‘fragile’ so to speak, than the LifePO4s which are far more resilient, I think the AGMs are the better priority.
Is there anything obvious I’m missing here, or will this work?
Try using an automatic charge relay to charge either battery of the other - note to check the max system voltages for charging Li in parallel with AGM - the voltages must be compatible with both batteries.
?? AGM’s are more durable than Lithium. Li does not respond to over charge or over discharge as well as AGM.
Providing the charge voltages for the AGM battery are within those required for the LFP just fit a VSR to charge the AGM.
14.2v charge and 13.5v float should be fine for the AGM the only problem could be that the Absorption period might not be long enough for the AGM but it’s better than you have now.
Hmm OK, so if AGM is more sensitive than Lithium then I’ll leave the MPPT connected to the Lithium.
I was going to just put a three way switch between the MPPT and the two battery types, and just change profiles in the MPPT when I switched over, so each was properly charged, but problem is there just isn’t enough solar to go around. I need more energy from alternator and shore power to keep them topped up.
So I guess a VSR sounds good if I can get it set up right. The most popular models are rated at 140 amps. Is there a way to limit that to around 50 amps? So I don’t accidentally kill my alternator? And would I need 140 amp cables? Those would be monsters and probably cost a fortune I imagine. But again I’m probably missing something
Just to clarify, I should leave the MPPT connected to the Lithium and put a VSR between the two batteries. So I’m not using the MPPT Load output at all? Couldn’t the MPPT basically act as a VSR if setup as I described? Or am I not understanding the Load function properly?
Just connect the VSR between the two batteries with the sense side on the LFP. The cables should be 16mm or thereabouts enough to take the full solar output and a bit more.
The VSR is just a switch so can’t as such overload the alternator, however to stop the LFP running the alternator hard just put a NC relay controlled from the ignition switch in the neutral to the VSR. This would switch it off as soon as you turn the ignition on.
OK, so the VSR is actually one way? So this would charge from the Lithium to the AGM only?
When you say 16mm you mean diameter or mm2? I have 180 watts of solar currently going through a 10 AWG cable. OMG all these different standards can be dizzying LOL
So, wait then how would my Lithium get charged when the alternator or shore power are on? This is also a big problem as the Lithium gets no help at all except solar so a lot is lost when engine is running or on shore power and I’d like to ‘grab’ some of that wasted energy.
Finally, well OK, I’ll google NC relay and get back
They are two way, its a mechanical relay, once its closed theres no way of controlling the power flow. Be careful with that.
Since you also dont have a way of charging the house battery from the alternator, the best idea would be to use an Orion smart between starter and house battery, then connect the MPPT to the starter battery. So all your charging sources go to the starter battery, and the house battery is charged through the Orion, no matter if the power actually comes from the MPPT, alternator or shore.
Im using a similar system in my 4x4, since i want to make sure that i can start the engine. By connecting the chargers to the starter battery, it gets priority. Only if the starter battery is charged then the Orion (in my case a relay since its both lead-acid) will also charge the house battery which is secondary to me
Once the charge voltage goes above the preset usually 13.4v it switches on and charges the other battery.
Only reason for the NC relay is the LFP battery’s will take a lot of charge from the alternator and if running for a long period could cause it to overheat.
Be careful when using a VSR about connecting charge sources to the starter battery rather than the domestics.
If the domestics are low and the starter fully charged then when the VSR switches in there will be a lot of current transfering from the starter to the domestics probably over the capability of the relay. The same obviously applies if the domestics are fully charged and the starter is flat, however if the starter battery is just that and not running other services then they rarely are below 90%.
I can’t think of a use for the load output it’s limited to about 10amps. However I use mine to charge the MiFi that allows me communication with the system when I’m not there.
Yeah I really want to buy an Orion XS (the new Smart) when the budget fits. But it’s hard to justify now considering it has no two way charging and the Renogy B2B does have this feature at less than half the price. It would be nice because that would solve everything. Charging both batteries from shore, and alternator, and solar- and one piece of hardware that fits on my boat, wonderful. Unfortunately there is no expected date for two way charging and that keeps me from kicking for the Victron. I think a lot of people find this a glaring omission in the Victron unfortunately, esp. since they say the hardware supports this feature. Please Victron hire another developer
If you use the NC relay then it goes in the neutral to the VSR ( the thin wire) it allows you to switch off the vsr when the engine is running thus protecting the alternator.
After reading other bits you have written about wanting to charge the LFP battery from the alternator I don’t think this will work for you.
