Need to upgrade solar system please help

[rvdrifter]

Mobile system on a fifth wheel trailer. I currently have 2 100W flexible panels with delamination at least 5 years old, 3 12V 105AH lead acid batteries 3 years old, pwm controller. I want to upgrade to lithium with new batteries, new panels, and a new controller. I am on a budget and need to purchase items separately. Also my rig has a charge converter when connected to shore power.
I recently purchased 2 100AH Weize batteries with a maximum input of 100 amps. Not connected yet.
Question 1: Can I use my current Powermax PM4-100 Charge converter? This is a 100amp charger. It does not have a lithium setting. It works in 3 stages starts at 14.6V and steps down to a 13.6V float. There is a way to set it for a fixed voltage up to 16.5V so I can set the fixed voltage to 14.4V.
Question 2: Is there a way to limit the amperage input to the new batteries to allow a slow charge and avoid triggering the BMS? I was trying to find a device that will limit the output of the Powermax PM4, but I'm not sure what I'm looking for.
I think I need 200W of solar per battery for solar charge, but I am limited on space to install due to vents, air conditioning, satellite, etcetera. Any suggestions?
Thank you

 

[Vigo]

Welcome.

So to clarify, the Weize batteries are lifepo4? They sell both lithium and lead acid 12v 100ah batteries, so want to make sure im reading you correctly.

Your converter does have a fixed voltage mode. There is a switch and potentiometer (adjustment you spin with a screwdriver) on the side of the case. I found this in the manual:


As for limiting power into the batteries, you don't have to if you don't want to. The converter has a max output of 100a, which means if the 2 batteries are wired in parallel with equal resistance cabling paths, they will each get 50a max. That is not a bad number and does not need to be 'protected against' per se, but you may choose to set it to a lower current than that to maximize the life of the batteries, since charging them aggressively does affect their lifespan somewhat.

There is no direct way through just the converter or the batteries to put a hard limit on charge current. Lifepo4 has a very low internal resistance so any time the batteries are more than a few tenths of a volt below what the charger is putting out, it will probably max out on amps. You could install another device to try and control this but since 50a/each is not a terrible number, you have the option of mostly ignoring this issue if you chose to. The situation i would personally be most concerned about is one battery disconnecting from the circuit for some reason, would allow the other one to get close to 100a for longish period. Of course they have BMS to hopefully disconnect them if they get into a bad situation, but it would be better not to rely on them. Having the converter set to 14.4 does protect you somewhat because even if the BMS did absolutely nothing, you wouldn't be over-charging the batteries except in case of cell failure, which in the case of a sealed battery is a total battery failure anyway unless you want to do DIY surgery, so overcharging the other cells in that situation may or may not make anything any worse anyway.

If you choose to install another device to try and control current, you get into some weirdness. I have used a large 80A pwm charge controller with a selectable current limit, with a converter/charger as its 'solar input', to charge lithium batteries. That would essentially have the lithium charge profile (or custom settings) in it and allow you to set a current limit between the converter and the batteries (as well as give you a load port with custom settings you could use for something somehow). It's an 'off label'/not-manufacturer-approved use and i think a lot of people here would try to convince you to do something else. But it's an option.

Another option is, given that those Powermax converters are <$200 on amazon, just to buy one of a lower current limit that doesn't need to be 'controlled' in that way because it can't possibly over-current the batteries. The issue there is just the thought of running a smaller device at 100% vs running a larger device which you already have, at 50% or something like that. One may have better longevity.

As far as fitting more solar, one upside of lifepo4 batteries is they don't mind if they don't get fully charged daily, or even often. They can sit with partial charge just fine. So if you can't fit more solar, that's fine, but if you are using more electricity than you're making with solar on average, you will have to charge with shore power or generator every once in a while to break even again and keep the batteries from eventually draining entirely.

 

[rvdrifter]

Thank you,
Yes, the new batteries are LifePo4. It sounds like; if I want to add a limit device for protection, the solution might be to add a 3rd LifePo4 battery when I can.

 

[Vigo]

I was tempted to say that, but since you said the budget was limited I didn't want to sound snide. But yes, by the time you get a 3rd battery in there they would all be quite happy with ~33a max charging. If you wanted to protect against each battery having too much current for too long of a time (because one or more disconnected by their bms for whatever reason), you could install circuit breakers in line with them. Could do something like 3 ~50a self-resetting thermal breakers in which case one battery dropping out would cause the other two to cut in an out erratically, or run all 3 positives or all 3 negatives through a 3-pole circuit breaker, in which case if any single battery tripped the breaker from over-current, all 3 would be disconnected until you got to them. Optional steps, just throwing options out there if you were concerned about it.

 

[fratermus]

Your converter does have a fixed voltage mode. There is a switch and potentiometer (adjustment you spin with a screwdriver) on the side of the case. I found this in the manual:
...
As for limiting power into the batteries, you don't have to if you don't want to.

It might be worth considering setting the converter pot even lower, to something like 13.6v. This would be a fine perma-float for LFP and would also reduce current.

 

[Vigo]

The only big downside to that is it would rapidly charge from wherever you start up to maybe 13.3 or so, and then be extremely slow charging past that. So you would not be able to rapidly recover most of the battery that way. Might get from 0-50% pretty quick but then slow down, etc. Just ballparking that. It takes a few tenths of difference between charger and battery to charge at 'full speed'. Some chemistries won't even do anything when hooked to a .1-2v difference.. i think lifepo4 does, just slower.