Question about 12v li-ion 3s battery for RV.

[jasonahf]

I understand that Li-ion 18650 batteries do not have a configuration to perfectly match lead acid battery voltage.
3s effective range 9v-12.6v, 4s effective range 12v-16.8. and Lead acid at 10.5v-12.7
3s might get over charged if bms fails to cut off leading to catastrophic failure, where as a 4s battery would never never be more than half charged (3.6-3.7v per cell)

Lets say I was to make a 3s36p battery with a high quality BMS that I would trust to cut off the charge voltage to the battery when the cells are fully charged.

Can anyone see any other issues that might arise from the lower voltage end or any experience as to what might fail to run at the lower end of the battery voltage? (Excluding 120v inverters as I don't use them)

 

[stan.distortion]

Iirc 4s LifePo4 gives a closer range, energy density isn't as good as Li-ion but lifespan and discharge rates are generally better. You're running the lithium cells alongside lead acid? If not, BMS voltage limiting could be an issue, alternators burn out pretty much immediately if disconnected from the vehicle battery when under load.

 

[Hillhater]

I assume you are talking of charging from the engine alternator.. ?
Why not just use a dc/dc charger which is designed to correctly charge to the correct voltage for the pack

 

[jasonahf]

The reason for wanting to do it with 18650's is that I have hundreds of good 3000mah cells. Lifepo4 would be better but I'm more interested in find a use for what I have.

Would not be using as a vehicle battery... it would be for my camper/trailer. So the 2 main sources of charge current would be the towing vehicle alternator (which would have it's own 12v car battery) and the built in inverter unit in the trailer which provides a 2-4 amp charging current when it's plugged into shore power (120vac)

 

[stan.distortion]

That's a 12v inverter and if so, any idea what its low voltage cutoff point is? Could be it will work fine with 3s and you're going to need to limit charge rate from the vehicles battery anyway. Not sure what the options are for 12v(+) to 3s charging, RC balance chargers would be one possible option, not very high charge rates but they're dependable, only cost around 30 euro and are available dual supply (12v DC and 120/240v AC) with 1s to 6s output.

 

[ZeroEm]

Using 18650's could just think about higher voltage pack. you going to need to control everything that goes in and goes out anyway. then you could get into off grid type equipment.

 

[amberwolf]

Yes; if you want to use the full capacity of the cells, you might want to build a powerwall type of setup, with something along the lines of a solar-like charge controller that boosts voltage from the 12v system to charge the powerwall whenever the engine is running, and when it is not running it bucks voltage from the powerwall to the 12v system.

As for using the cells directly, back when I still had St Bernards that would travel around with me (Kirin and Yogi did, but Jelly doesn't) I had begun to work out using some EIG NMC 20Ah cells to power a cheap 3kw 12vdc-110vac inverter to run a small window AC unit in a custom insulated trailer for the dogs.... Like you've found, I'd be able to use only a portion of the pack's capacity, probalby less than half, more like a third in my case, IIRC.

I halted work on the project until I could come up with a better way, but didn't while Kirin and Yogi were still around; I think the idea at the start of this post would work but never worked out the details.

 

[jasonahf]

so yeah.. when the trailer is towed it'll see 14.4V from the vehicle alternator, and when plugged into 120vac the built in inverter provides up to the same for all the 12v accessories and to charge the batteries. Not too worried about limiting amperage to the battery because it'll be around 35P. The cells have tested fine on a high speed charger putting 2amps into the cells, so could handle up to 72 amps but that'd probably melt wires first LOL.

Either system though would only be supplying around 10 amps max. And as long as I use a really good BMS it should cut off the batteries to prevent over charging.

I was more curious if anyone's had experience with how the 12v accessories might react when the voltage dips down to lets say around 9V, whereas a Lead acid battery normally be dead at 10-10.5V.
Like the water pump, furnace (with igniter) and the electronics for the 2 way fridge etc.... that kinda thing

 

[amberwolf]

Depends on the device; it's something you can test easily enough if you have a variable power supply capable of supplying the current any one specific device needs.

In my general experience with "12v" devices intended to run from a "12v" battery, they operate less and less well the farther below about 11-12v they get.

Some stop working well before 10v. Some that are simple resistive loads (like many lights can be, and some motor-powered loads like fans) may operate down to the point that they simply can't do useful work..

The more complex the device the more likely it is to have a power supply that has a hard cutoff somewhere but that can boost a low voltage to keep going at it's normal workload right up until the cutoff. These draw more and more current (causing more and more voltage sag) as the voltage decreases, to make up the same wattage output to the device.

There are certain complex PSU designs that may fail if the input drops below a certain point, and/or don't gracefully handle brownouts or sustained low-voltage. So it is (rarely) possible to damage or destroy a device by using too low an input voltage. I don't know an easy way to tell this might happen until it's actually happened.

 

[jasonahf]

I guess I could also rethink this and go with a 24v battery, using a BMS with a separate charge port.

Could isolate the output from the inverter and the truck alternator and connect to the charge port through a step-up converter.
Then connect the output from the battery to the RV through a step-down converter.

A bit more work but should accomplish what I'm looking to do.

 

[gobi]

I guess I could also rethink this and go with a 24v battery, using a BMS with a separate charge port.

Could isolate the output from the inverter and the truck alternator and connect to the charge port through a step-up converter.
Then connect the output from the battery to the RV through a step-down converter.

A bit more work but should accomplish what I'm looking to do.

I would not make a battery based on vehicle voltage or conventional 12v or 24v number.

First pick your inverter that will be used to convert the DC voltage,
Check the voltage range of that product.
Say the upper range for that inverter is 30v, so that gives you 7.5s, you can safely build a 7s battery with x cells in parallel,

Then pick you charging solution that you able to set the charging voltage and set it to 4v per cell and forget about it,
I like a JDM bluetooth based bms so I can monitor the bank voltages.

 

[john61ct]

You need DC-DC conversion trying to do 12V.

Adds to both cost and complexity, reduces reliability increases fire risk.

Best to go to 7S 24V is common.

But really LFP is the way to go.

Would you heat your mobile living space with kerosene just because you had a lot sitting around?

 

[DingusMcGee]

I had a few hundred 18650 cells and wanted a useful way to employ them in an RV. The Magnum Dimension CSW 20-24 inverter can use either 12v or 24v as input. The upper voltage limit for when the inverter is set at 24v is 30 v. So I made 7s packs. 7 x 4.2 = 29.4 volts < 30v and the inverted works fine.

The Morning Side solar controller is adjustable for various charging configuration and works with these batt packs. For 12v needs I have a 90 amp 24v to 12v DC down-converter.

LFP cells cannot safely be charged at low temps and therefore are not always suited as best for such applications.

john61ct, I suppose the OP person wants to recoup his present worth on those 18650 cells — seems reasonable?