Combine lithium battery bank with AGM bank?

[Zardor]

Hello, I have two 100 aph lithium batteries. I'm finding that with my cpap machine and refrigerator (along with watching tv at night) are dropping my battery bank level really low. I wake up with about 25-30% battery life remaining - I paid extra for the lithium so that I could draw more than 50%, but my concern is dropping below the 20% maximum with this batteries when my solar wasn't able to get me back to 100% before the next evening.

My question is can I add an AGM battery bank as a back up? I can't afford another lithium battery, but can afford an AGM.

 

[John61CT]

As a separate bank you switch over to, not combined.

How are you measuring SoC now?

First focus on the issue, no matter how big your bank

You must replace the AH you take out each day, with what you put in, plus 20-30%.


What model fridge?

How efficient is your TV setup?

Are inverters involved anywhere (not good)

Invest in a Watts Up and come back with hard numbers

Amps while running, AH consumed per day

for each load device.

And, most importantly, what is your daily source of charging?

 

[Zardor]

I have just over 1000 watts of solar on three huge panels (Yes, securing these was a nightmare), they recharge the batteries completely everyday - i dont know how fast they do this - never thought to check on it before i use it. My concern is when it's overcast. But I figure6d out an alternate solution. I'm just going to take one of the solar panels, direct it to its own separate battery bank. It will take care of my cpap only - which naturally was my biggest concern. This will allow me to use my larger setup without concern. Thank you for your quick response I really appreciate you answering me.

 

[John61CT]

That would work. Adjust the charge profile / setpoints for each bank appropriately.

Specs on the panel?

But I think reducing consumption is also still a good idea.

 

[Zardor]

I think they are 360 watts each. I would need to look for the invoice for the exact figure. I know it sounds excessive, but I'm an independent graphic artist, I have a large energy requirement.

 

[John61CT]

I think they are 360 watts each.

Sorry, what are?

 

[Zardor]

Each solar panel

 

[John61CT]

Ah.

Voc and Imp?

 

[Zardor]

Now your speaking a different language. When it comes to solar I'm a complete novice. I called az wind and solar, explained what I used on a daily basis and they put the system together for me. However, now that I know a little more I don't think I would have went with such large panels.

 

[John61CT]

This is the stuff you need when matching a SC to a panel.

The web page with your model listed has the specs.

So should a sticker, maybe a paper included when you unpacked them.


Voc is "Open Circuit" volts

Impp is amps at "Max Power"

_______
A sample list:

Module Watts STC: 265 Watts
Module Watts PTC: 241.00 Watts
Max Power Voltage (Vmpp): 31.40 Volts
Max Power Current (Impp): 8.44 Amps
Open Circuit Voltage (Voc): 38.60 Volts
Short Circuit Current (Isc): 9.03 Amps
Max System Voltage: 1000.00 Volts
Series Fuse Rating: 15 Amps
Module Efficiency: 16.19%
Temp Coefficient (Pmpp): -0.4000%
Temp Coefficient (Voc): -0.3000%

 

[John61CT]

What controller did you get?

 

[John61CT]

What vehicle and/or trailer?

 

[John61CT]

What is the highest voltage the controller gets up to before going to Float, IOW what is Absorb set to?

and what is Float set to?

Do you know the logic behind how long the SC holds Absorb before going to Float?

 

[John61CT]

What model is the LFP battery?

 

[John61CT]

now that I know a little more I don't think I would have went with such large panels.

No, I think the solar part is ideal.
You just need more storage, and I think if your answers to the above work out, we'll find a cheap and simple solution, manual for now if you can't afford a fully automated one.

Plus make sure your LFP are set up to last as long as possible. Is there an external BMS on those 100AH packs?

 

[akrvbob]

I think you've already got the perfect solution. Add a separate battery bank and dedicate one of the panels to it. Then use it for just one purpose, my first guess is that the CPAP would be ideal. Just be sure the battery can safely handle that draw every night. With little else using it during the day it should always get back to 100% and you can use it to take some of the load off the other lithium battery bank.

Call Nothern AZ Wind Sun and get their advice on the controller and batteries.

It's highly unlikely that you will regret having that much solar!!

 

[willprowse]

Actually, lithium and lead acid combo is fun to do. Because lead acids have great discharge rate for large loads. And lithium lasts many years. Best combo for this is to get an inergy kodiak and use their expansion terminals to add a few lead acids. Works nicely. I mention this in a few videos and on my website.

 

[John61CT]

LFP has **many times** higher discharge rates than lead, and with much less voltage drop, important for high amp inverter usage.

Kodiak to me is a little portable unit, not big enough for most house loads. Are they even LFP or LiPo/NMC?

And mixing lead and LFP will not be good for one or the other, each needs their own charge profile.

If longevity is your goal.

 

[willprowse]

LFP has **many times** higher discharge rates than lead, and with much less voltage drop, important for high amp inverter usage.

Kodiak to me is a little portable unit, not big enough for most house loads. Are they even LFP or LiPo/NMC?

And mixing lead and LFP will not be good for one or the other, each needs their own charge profile.

If longevity is your goal.

All depends on the design of the lithium battery and its chemistry. and sure it has way higher discharge rate than lead, for the amp hour rating of the battery, but not for the price. I can get a 35 amp hour battery to discharge 300 amps no problem. sealed agm deep cycle too. and that is only 135 bucks. A lithium iron phosphate 100 amp hour battery can discharge 100 amps usually (some more some less, depends on design, which means individual discharge rate of the cells). And that battery would cost 899 bucks+, usually 1200 bucks. No comparison.

Kodiak can be used for house loads. check it out. and its a lithium ion. panasonic cells I believe.

and yeah of course, you would never ever mix the batteries!! One would charge way to fast after first large discharge taken over by the lithium because of low resistance. But if you have a battery charger separating the lead acid, its totally fine. Really depends. Its totally possible to use both and still isolate them. Takes a bit of work. Unless you use the kodiak. It kicks butt. Hook lead acids straight to it. has its own charging and discharging circuit for the lead acid maintenance. its cool!

 

[John61CT]

The term "lithium ion" means very little, just an umbrella term for many different chemistries.

LFP (LiFePO4) is the only one of them long-lived enough and safe from thermal runaway (boom bad!) for fulltime House bank usage in a van.

Anything you can pick up and carry around will only be suitable for a little fan and recharging a few screen gadgets. Most of those are NMC or LiPo, very short lived, susceptible to fire risk.

Often replacement cells not even available, very overpriced and grossly exaggerated capacity: compare at standardized 12V AH capacity at the 20-hour rate. Usually quoted in wH, or mAH based on the voltage of the internal component cells, very deceptive, oftem impossible to get accurate specs

Try to get a link to replacement cells.

Post a detailed review, tech specs overview of the Kodiak, so we can investigate further. I'm sincerely very interested in drilling down into the details.

Ideally with a knowledgeable owner will participate, with an accurate DMM and ammeter so we can get objective info and properly analyse its capabilities.