VSR instead of B2B in LiFePo / lead acid hybrid system?

[ckris]

That's a very small battery. You probably get away with it because you have a relatively large alternator and small battery. My lifePO4 battery is 560Ah in 12v money (24vboat) the amount of amps that battery would pull is considerably more.
LifePO4 has better charge acceptance but in reality some of your alternator output is going to heat the wire.

Haha, yes size matters! But the op was talking about adding 50ah lithium.

The point is, lowering the charge voltage does reduce the current accepted and this will be true for any size of battery (even yours if you lower the charge voltage enough). The voltage drop along battery cables may do this sufficiently to prevent alternator being overloaded in many installations.

 

[geem]

Haha, yes size matters! But the op was talking about adding 50ah lithium.

The point is, lowering the charge voltage does reduce the current accepted and this will be true for any size of battery (even yours if you lower the charge voltage enough). The voltage drop along battery cables may do this sufficiently to prevent alternator being overloaded in many installations.

And there are dozens of stories on the Internet of burnt out alternators. Did you work out the resistance of the battery cable, the length of the cable and do a volt drop calculation or just wing it?

 

[kwb78]

Yep if it works OK that's great.

Now of course you must have a charging voltage in excess of the inherent cell voltage and as soon as you exceed that charge voltage current can be quite high in a very non linier manner. (a bit like an LED diode) However it still seems to me that reducing charge voltage will reduce charge current and if this is done by series resistance or a droop in alternator voltage then a balance of charge voltage and current must be achieved. ie your graphs show nicely the cell voltage with constant current. I am talking about not constant current. ol'will confused. Perhaps others are also.

I think the confusion is because of the difference between a constant current and a constant voltage source, and what is causing the voltage to be lowered. An alternator without a load is a constant voltage source - the output voltage is regulated to be a fixed value somewhere around 14.5V. That means that when you connect a load across it, the load will see 14.5V and current will flow. The resistance of the load will determine how much current flows. If you connect a 100W lamp across the terminals, you will see a load of about 6.9A because the lamp will have a resistance of about 2.1Ω. This is way below the capacity of the alternator, so all that happens is the regulator will increase the output a bit and you will still measure 14.5V at the output of the alternator.

If instead you connect a very low resistance load like a dead short, the current will be very high. This will cause the regulator to reach its maximum output and be unable to maintain the voltage at 14.5V. So what will happen is that the measured voltage will drop to some lower value, but the current will remain at the maximum. An LFP battery is not far off being a dead short load, because the internal resistance is extremely low, of the order of 1mΩ or less.

An LFP battery will be around 3.4V per cell (13.6V for 4 cells) for the majority of its charge. With an alternator that can supply unlimited power at 14.5V, the 0.9V differential will result in a current of about 900A. That's obviously way higher than most alternators can supply, so what happens instead is that the regulator tries to maintain the 14.5V output but is unable to and the voltage measured at the output becomes lower. In practice it will reduce to give a differential at the battery that will result in the alternator's maximum current flowing. If we take our battery at 13.6V with its internal resistance of 1mΩ, the voltage differential that will give for example 80A is 0.08V, so the alternator voltage would drop to 13.68V. The alternator is now operating as a constant current source.

As the battery charges, its voltage will gradually increase, but for an LFP battery which has a very flat charge curve, the voltage doesn't change very much over the whole charge. As the battery gets close to being charged the voltage does start to rise more quickly, and the difference between the alternator's desired 14.5V and the battery voltage is small enough that the alternator starts to reduce its output again to maintain the 14.5V. Obviously if the alternator regulator voltage is set higher, this happens later.

In practice, things like the BMS (especially if it's MOSFET based rather than relay) and cabling could significantly add to the overall resistance of the circuit, and that's probably why ckris is seeing more limited charging current. You could deliberately add a resistance to the circuit to limit charging current (and this would have the effect of lowering the voltage seen at the battery), however it's not a particularly efficient way to do it and is uncontrolled so you'd have to be quite careful about selecting a value.

It's also worth checking what the correct charge rate of the battery is - EVE 105Ah cells for example specify 0.5C for charging, meaning that you'd want to limit charging to about 50A for best cell longevity. The best way to do that is to use some form of active control.

 

[William_H]

Thanks kwb78 for your efforts educating me. I think my understanding is now approaching similar to yours. Thanks ol'will