Questions regarding batts and panels

[frater secessus]

Is your charging that simple?

Li charging can be simple to implement {after} one understands what's going on. For example "hold 13.2v forever" to maintain mid-SoC or "charge to 14.0v and stop" to get near the top.

Say just max watts of input (no limit)

The max input allowed for charging our type of lithium is typically 1C (100A per 100Ah of capacity), enforced by BMS.

The max recommended limit by most manufacturers is 0.5C, or 50A per 100Ah. The rate on which claims of "kerbillions and katrillions of cycles!!!!" are based are typically 0.2C, 20A per 100ah.

Pick your poison. My solar charging is typically <0.20 because it is overpaneled and charged to targets before local solar noon. If it's been cloudy all day then bright at noon it can charge at >0.4C. My alternator charging is around 0.3C.

In addition, the common wisdom that "lithium doesn't need absorption" is predicated on two factors:

1. mild charge rates like 0.2C. Above that some absorption will likely be required to hit SoC targets
2. charging voltages ≥14.0v. Below that some absorption will likely be required.

at <13.8v

I charge at 13.8v, 3.45 per cell, with ~20 minutes of Absorption duration.

then hold 13.8v when it's reached?

I typically hold 13.4v after charging; this allows gentle settling away from 100% SoC while preserving capacity.

 

[frater secessus]

My charge controllers wouldn't be charging the battery at more than 1 to 2 amps at 13.8 volts

Another datapoint: at 0836 local my bank is accepting 16.4A @ 13.5v.

 

[frater secessus]

For the curious, the Off-Grid Garage YT channel has done a lot of testing with charging, current rates, voltages, and absorption durations.

 

[user 423]

If it's not enough he's considering some kind of switch where only one battery is used at a time but both can charge at the same time. Anyone ever do this?

He's electrically inclined but solar is new to him. I know just enough to start a fire. Thanks in advance.

I'm wondering about this too since I want to add a 2nd. identical AGM to one that is two years old.

It would seem that you would need 2 controllers?

 

[user 423]

Our bank:
* six ancient Concorde Lifeline 105ah AGM.
New in 2002, two decades ago.
We started with eight, but one then another decided to retire... so, despite the 'forever' hype, they sometimes get all tuckered out.

It's nice to know that AGMs can last that long if kept charged properly. I do notice that Concordes are twice the price of run-of-the-mill AGMs. If you get that long out of them though, it's money well spent!

 

[Headache]

I'm wondering about this too since I want to add a 2nd. identical AGM to one that is two years old.

It would seem that you would need 2 controllers?

If you don't want the new battery to equalize down to the old then it seems to me you'd be creating 2 separate systems. If you can get a lithium battery you'll avoid that as you can add new to old without having to be concerned with it. The 200 amp hour batteries are as low as around $600 now.

We just went ahead and pulled the trigger for two 200 amp lifepo4 batteries. We're going to add more as we get more watts on the roof.