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Question on switching to LiFePO4 batteries.

Sterling1
Explorer
Explorer
I'm going to replace my 2 lead acid batteries in my Bigfoot truck camper with two 100ah LiFePO4 batteries. I've previously installed a 4 way battery selector switch to allow me to keep one of my batteries at a high enough charge level to start my generator (they are run in parallel). However, this won't be necessary (and probably won't work) with the lithium batteries and I will probably need to have it at the Both position to start my generator.

My question is is can I damage the batteries if I switch to the Both position when one battery is low and the other battery is fully charged?
35 REPLIES 35

mbloof
Explorer
Explorer
Sterling1 wrote:
I'm going to replace my 2 lead acid batteries in my Bigfoot truck camper with two 100ah LiFePO4 batteries. I've previously installed a 4 way battery selector switch to allow me to keep one of my batteries at a high enough charge level to start my generator (they are run in parallel). However, this won't be necessary (and probably won't work) with the lithium batteries and I will probably need to have it at the Both position to start my generator.

My question is is can I damage the batteries if I switch to the Both position when one battery is low and the other battery is fully charged?


In a word: No.

A battery is simply a energy storage device.

No matter what "type" or chemistry the batteries are if they have the same nominal voltage (IE: 12V) then the following would be true:

Battery #1 is 12.5V and Battery #2 is 13.5V and you hook them both together in parallel along with a load. What happens?

The load will draw current from BOTH batteries but since Battery #2 is at a higher charge voltage than battery #1 some of Battery #2's energy will flow to Battery #1 until such time BOTH batteries have the same charge voltage.

If you simply hooked a not-so-much charged battery up to a fully charged battery without a actual load the "not-so-much charged battery" would still be a load to the fully charged battery until such time that both batteries have equal voltage.

So can you can see the 'wasted' energy/charge being used for the lesser charged battery?

This is why it is generally recommended that to use two or more batteries in parallel they ought to be the same Brand+type+age+capacity and charge level so stored energy is not wasted charging the other batteries in the bank.


- Mark0.

3_tons
Explorer
Explorer
A common shunt size is 500a, but they also come in larger sizes depending on whatever
your needs…Also, some manufacturers SOC panel meters use a proprietary meter connector, so if you plan on a particular meter later on, get the shunt that’s compatible, or simply choose a shunt with a Bluetooth feature.

As to your second question, yes one battery will try to charge the other battery, so IMO best to keep them in a parallel config…

Upon edit, I would also add that if one battery were to charge another, the shunt meter would not measure this activity, which will result in unwanted inaccuracy, and in a similar sense, trying to determine SOC of an LFP battery by measuring voltage alone is impractical, only slightly better than a WAG (wild arse guess)…LFP’s are not cheap, and they deserve proper care…

3 tons

Sterling1
Explorer
Explorer
Yea, it seems like there won't be much use for the 4 way switch other than isolating the batteries for storage and individually charging them if they get out of "balance".
I plan on just using a voltage meter to determine SOC for the time being, I realize it won't be very accurate but that will be OK for my use. But that does bring up another question. If I were to install a shunt based meter how is the generator handled? Can the shunt handle the current draw of the generator and not cause too much of a voltage drop across it?
And this brings up another question. My generator and inverter are connected directly to the batteries and don't go through the switch so they only see current from both batteries if the switch is set to Both. This didn't cause any problems with my current setup but if I inadvertently tried to use them without the switch set to Both it wouldn't cause any strange back feed issues would it?

3_tons
Explorer
Explorer
With LFP’s you’d be better off to just leave them in continuous parallel, no longer a need to isolate one…

Charge them both to 100% before putting them in parallel…

I will add here what I’d penned in another thread awhile back, but take note of item #2, because determining state of charge (SOC) with LFP is problematic due to it’s relatively FLAT discharge curve (meaning relatively near-stable voltage):

*************

Well, since (your camper) spends much of it’s life plugged in, you might then consider doing what I and many others with LFP’s do…

1) Simply add an automotive ‘side-post’ battery knife style disconnect switch to the LFP’s two terminals (the most accessible), so that the battery can hibernate isolated when not in use…Once isolated, the self-discharge rate for LFP’s is practically non-existent, say over a 6 month period. Hibernation should be done with the battery at about 50-60’ish% state of charge (SOC), this to keep LFP ions in appropriate equilibrium…

2) To best determine ‘accurate’ SOC’s with LFP’s (otherwise somewhat problematic), you’ll want a good LFP compatible, shunt based SOC meter, like Victron’s ‘smart-shunt’ with Bluetooth (simply installs in NEG battery cable)…Reads like a gas gauge and provides SOC, volts, amps and history…

3) With battery disconnect during these periods of ‘non-use’, your existing on-board converter-charger (via shore power) will take over and continue to supply 12v power to the camper…

4) When restoring your batteries back to a full charge, most any OEM charger that’ll output 14.X volts (not to exceed 14.6v) will do - however, the key here is, once the batteries are fully charged and *balanced (say, another 15-20 mins after achieving 100% SOC…), discontinue charging (this, in the interest of battery longevity), by shutting off your generator (or if shore power, via on-board converter-charger’s breaker), thereafter running off battery power…

* Note, the cell balancing cycle can be observed via Bluetooth per the slight cycling of amps and volts until such cycling stops (see item 2).

5) Unlike wet-cells, there’s no need to ‘routinely’ restore LFP’s back to a 100% FULL charge…85-95% is quite sufficient thanks to LFP’s far deeper DOD (depth of discharge) - But to aid with battery longevity, you’ll only want to do a FULL charge occasionally, and for just two reasons:

a) Periodic cell re-balancing (this process begins after achieving 100% FULL Charge).

b) Meter resync (concurrent with a full charge) - to resolve cumulative SOC meter drift which naturally develops over a time…

6) At this point you might sense (as with other type batteries) that solar harvest will help minimize the frequency for having to run your generator, as does LFP’s deeper DOD…

3 tons

Sterling1
Explorer
Explorer
Dr. Prepare (made by Renogy) 100ah, 100 amp max continuous discharge rating, 50 amp max continuous charge rating. I saw that warning about about voltage when connecting some LifEpo4 BATTERIES in parallel but there is no warning about different voltages in the battery manual.

KD4UPL
Explorer
Explorer
Hard to know since you don't specify the battery. Generally Li batteries warn you to only parallel them when the voltages are very close.