LIFEPO4 Cheap

[Roll With The Changes]

Let me try to help you understand the "Parallel" Battery and Converter/Charger Issue...

First, it helps to not think of the battery/converter connection as a parallel connection, think of it as a "Common", Buss Bar type connection. For example: In my coach the battery is directly connected to a Buss-Bar (after the fuse) then the Converter/Charger is connected to the same Buss-Bar, additionally, the main line that powers the coach accessories is connected to this same Buss-Bar. This configuration creates a "Common" connecting point versus what you are considering a "Parallel" connection. All voltage inputs and accessories/loads are connected to this common buss-bar (PS: this is also where you connect your Solar output to as well). What happens in reality is that electrons flow freely to wherever they need to go, i.e.: if your converter is outputting voltage and you open your slide, the electrons from the Converter are going to flow to the slide motor(s) and if the converter is not producing enough amperage to fully supply the slide, then your battery will provide supplemental power to boost what the converter can supply. Once the slides stop, the electrons from the converter will begin to flow into the battery to replenish whatever electrons it just supplied for the slide operation.

As far as your concern about the converter "rarely doing the right thing for the battery", the Battery Management System (BMS) inside the battery will automatically control the electrons coming into the battery and will accept as much as it can until the battery is charged and then stop electrons from entering the battery. What your converter does is produce a voltage (13.6 volts DC) the battery accepts all the electrons it can up to that 13.6 number (Lead Acid, or Lithium) once the system reaches equilibrium, the Converter reduces the charging voltage and goes into "Float" voltage 13.2v. This is where the problem with Lithium batteries comes into play because the voltage of Lithium batteries is (almost) always above 13.6 volts which causes the Converter to drop into "Float" mode (13.2v) and your Lithium batteries only get charged up to 13.2 volts versus 14.4v (+/-) when they are fully charged.

Your other concern regarding the braking from the truck and having a higher voltage is not an issue because the power for the brakes comes directly from the truck and does not go through the coaches 12v electrical system. The only time the coaches 12v electrical system comes into play is when the emergency brake lanyard is pulled, then the 12 volts for the brakes comes directly from the coach battery. otherwise there is no interaction with the trailer brakes.


PS: Congratulations on making the leap to Lithium Batteries, they are far, far, superior to Lead Acid batteries for RVs.

Thanks for the input. I'll try to ask my question in a little more detail. Let's try this scenario: I'll start with lead acid because the damage is more obvious. Starting with a fully charged battery, you plug in to shore power and the converter starts up in bulk mode at 14.4 volts. You open the slides which the converter sees as max current even though the battery only discharges at say 30 amps for one minute. When the slide operation is complete, the battery will return to full charge very quickly, but the common buss sees the house loads and stays in bulk mode for four hours until that cycle times out. Then the converter goes into absorption mode at 13.6 volts indefinitely because it never sees the current drop below 5 amps indicating that the battery is fully charged. The result is a regularly overcharged lead acid battery.

It seems to me that the only way for the converter/charger to operate intelligently is to measure charge current separately from the common buss.

Now the Lithium scenario: It looks like the PD and Victron chargers have a similar algorithm but they oper at the higher Lithium voltages. However, they still have the same limitation of measuring common buss voltage. This should not be a problem because the internal BMS will stop charging when it needs to. All that being said, why not just set converter output voltage to 14.6 and let the internal BMS manage the charge?

[Roll With The Changes]

@SolarPoweredRV

Your answer to the trailer braking problem is right on. I wasn't looking at it that way. But what happens when the breakaway is pulled and brakes are applied in the truck? Could the higher Lithium voltage cause a problem in the truck?

I realize this would not be a normal scenario, but the breakaway can get pulled accidentally. It has happened to me a few times and may happen again even though I think I fixed it 😁

[rootusrootus]

But what happens when the breakaway is pulled and brakes are applied in the truck? Could the higher Lithium voltage cause a problem in the truck?

Couple things...

First, a lithium battery that isn't actively being charged is going to rest at around 13.5 volts. The alternator on your pickup should be putting out around 14 volts. So the voltage from the lithium is within the bounds of normal for the 12V system on the truck.

Second, any modern brake controller has protection against back feeding power into it. The brakes themselves aren't directly connected to the truck's 12V system. Though I think you'll find it commonly recommended to disconnect the brake controller anyway if you're going to deliberately test the breakaway switch.



On a completely unrelated note, since this post made me think about it -- I had occasion a couple weeks ago to tow my trailer with a seriously depleted lithium battery, and I log my power usage & generation. I can say that a current model Ford F250 will not burn out the alternator trying to feed a hungry lithium battery. It capped out at about 85W from the truck.

[rootusrootus]

Thanks for the input. I'll try to ask my question in a little more detail. Let's try this scenario: I'll start with lead acid because the damage is more obvious. Starting with a fully charged battery, you plug in to shore power and the converter starts up in bulk mode at 14.4 volts. You open the slides which the converter sees as max current even though the battery only discharges at say 30 amps for one minute. When the slide operation is complete, the battery will return to full charge very quickly, but the common buss sees the house loads and stays in bulk mode for four hours until that cycle times out. Then the converter goes into absorption mode at 13.6 volts indefinitely because it never sees the current drop below 5 amps indicating that the battery is fully charged. The result is a regularly overcharged lead acid battery.

It seems to me that the only way for the converter/charger to operate intelligently is to measure charge current separately from the common buss.

It uses both voltage and current. Something consuming power drags down voltage, but a battery pulling the same current as its charging will be raising voltage. Between the two you can identify a stopping point. So the charger caps at 14.6V or so, holds that for a while while the current drops way off, and then releases the voltage down to about 13.5 (how long it holds the peak voltage is configurable, typically 30 minutes for a 100Ah LFP).

why not just set converter output voltage to 14.6 and let the internal BMS manage the charge?

You probably could, TBH, with the better BMS's.

[Roll With The Changes]

Couple things...

First, a lithium battery that isn't actively being charged is going to rest at around 13.5 volts. The alternator on your pickup should be putting out around 14 volts. So the voltage from the lithium is within the bounds of normal for the 12V system on the truck.

Second, any modern brake controller has protection against back feeding power into it. The brakes themselves aren't directly connected to the truck's 12V system. Though I think you'll find it commonly recommended to disconnect the brake controller anyway if you're going to deliberately test the breakaway switch.



On a completely unrelated note, since this post made me think about it -- I had occasion a couple weeks ago to tow my trailer with a seriously depleted lithium battery, and I log my power usage & generation. I can say that a current model Ford F250 will not burn out the alternator trying to feed a hungry lithium battery. It capped out at about 85W from the truck.

That's comforting to know. You have a good point about resting voltage but I'll disagree that the brakes are not connected directly to the truck. When the breakaway is activated, the trailer battery connects directly to the output of the brake controller in the truck. I like to think that the brake controller is smart enough to protect itself, but.... I do disconnect the truck when I test the breakaway, but accidents happen.

As for the dc/dc issue, the trailer charge circuit is fused at 30 amps, so I wouldn't think it would hurt the alternator and my 2020 F350 seems too smart for its own good anyway. All that being said, I will probably end up using a Renogy dc/dc with mppt controller. Having both functions in one unit is nice and it sets me up to install solar later.

I'm in a good position right now as we will stationary for a few months while I get this worked out.

[rootusrootus]

I'll disagree that the brakes are not connected directly to the truck. When the breakaway is activated, the trailer battery connects directly to the output of the brake controller in the truck. I like to think that the brake controller is smart enough to protect itself, but.... I do disconnect the truck when I test the breakaway, but accidents happen.

I think we agree. I was just saying that the brakes don't connect directly to the 12V bus in the truck, they go to the brake controller, which itself goes to the bus. So first the voltage would have to overcome whatever protection the brake controller has before it will have a shot at the rest of the 12V system on the truck.

As for the dc/dc issue, the trailer charge circuit is fused at 30 amps, so I wouldn't think it would hurt the alternator and my 2020 F350 seems too smart for its own good anyway. All that being said, I will probably end up using a Renogy dc/dc with mppt controller. Having both functions in one unit is nice and it sets me up to install solar later.

Yeah I think Fords are generally well regarded as having good protection for the alternator, but apparently some trucks don't, and can burn out the alternator trying too hard to feed the lithium. I've only heard that, however, not experienced it. Just thought it was interesting to quantify with my own measurements. When I got home I checked to see how much solar we got on the trip home and was surprised to see 225W (it's only a 165W panel, and aside from a freak day when we got 170W, it almost always gets more like 125-140W), then zoomed in and saw the spike when we turned on the truck and headed home. We weren't far from home, so a couple hours later I turned it off and it went back down to solar-only. Kinda fun to see the power flows.

[GD-Skip]

In addition to the very detailed explanation from David I would like to add:
The LFP prismatic/rectangular cells have a voltage range from 2.5 (discharged) to 3.65 (fully charged)
A “normal” Lead Acid battery charger should not be used and will never get the batteries full.
The charging process for LFP cells is CCCV. Meaning constant current - constant voltage. That is what they like.
A good LFP charger does it the following way:
It starts with an constant current (which can be adjusted. I use 10 % of the battery capacity) and charges with this constant current until the battery gets to 14.6 Volt. (That is the bulk charge portion) It measures the time for that period and then switches to constant voltage, which is the Absorption portion of the charge. Depending on the manufacturer of the charger, this absorption part lasts about 10 times the bulk charge time. (Max 12 hours at my charger)
During this time the current drops constantly to maintain this 14.4 Volt.
When this time is up, the charger switches to Float charge, which is about 14.4 Volt.
As has been said before, just measuring the voltage, what a lot of cheap chargers do, is in no way sufficient. Remember, the voltage difference between empty and fully charged is only 1.15 volt per cell.

[rootusrootus]

That is different from how I have my charger configured. Constant current until 14.5V, hold there for 30 minutes [per 100Ah of battery capacity], then float at 13.5. These values are configurable but are what is recommended by Battle Born.

[SolarPoweredRV]

Remember, the voltage difference between empty and fully charged is only 1.15 volt per cell.

When you think about it...

The new Tesla Model S, Plaid can accelerate 0 to 60 in less than 1.9 seconds and race up Pikes Peak in record winning time and it does all this with only 1.2 volts per cell.

THAT'S AMAZING !!!!

[SolarPoweredRV]

Remember, the voltage difference between empty and fully charged is only 1.15 volt per cell.

When you think about it...

The new Tesla Model S, Plaid can accelerate 0 to 60 in less than 1.9 seconds and race up Pikes Peak in record winning time and it does all this using only 1.2 volts per cell.

THAT'S AMAZING !!!!