Alternator with LFP: How to calculate current?

[Ulstue]

In practice, there are some people who charge their LFPs directly from the alternator without a DC-DC converter or charge booster or A2B/B2B charger.

According to these reports, it seems to me that most 80A alternators work permanently at their rated power, while 160A alternators also only provide 80A.

As far as I know, LFPs can also be charged at 1C - 5C depending on the manufacturer. However, this is then probably charged with up to 16V.

The question therefore: With how many A can a 14V charger (alternator) charge a LFP at a voltage difference of 0.8-1.2V?

If you are one who charge directly from the alternator: What is the capacity of your LFPs, what is the rated power of your alternators, and how many amps of charging power do you have in real terms?

How to calculate the possible charge current for a voltage difference of, for example, 1V (LFP 13.2V, alternator 14.2V)?

[Paul L]

Originally Posted by Ulstue

...

According to these reports, it seems to me that most 80A alternators work permanently at their rated power, while 160A alternators also only provide 80A.

.....

Not sure you mean what this sentence says,. Are you claiming that an 80 amp small frame alternator can deliver 80 amps in continuous service? They can't.

[Ulstue]

This is what has been written in various forum posts.... From people who reported, they have read that on their measuring equipment. Maybe 80A delivered from an 90A rated compact alternator.

Otherwise 2 x 120A alternators, catamaran 1400AH LFP, also only with 80A charging.


The question is: How many amps does an LFP draw at a voltage difference of 0.8V from a 14V alternator.
How do you calculate that?
How is this verified in practice?

[lmxr]

Originally Posted by Ulstue

In practice, there are some people who charge their LFPs directly from the alternator without a DC-DC converter or charge booster or A2B/B2B charger.

According to these reports, it seems to me that most 80A alternators work permanently at their rated power, while 160A alternators also only provide 80A.

Would you mind linking to a few of these reports? As far as I know, it is rather difficult to get more than 2/3 of the rated current continuously without overheating, even with heavy-duty alternators.



I'm aware that this does not directly answer your question, but I'm somewhat curious towards where you got this information from.

[wholybee]

I can get 90A from my 70A alternator, but only for about 10 minutes before it heats up and the regulator backs off. Temperature controlled it settles to about 40A.

I have seen a few reports of people using "stock" (which I take to mean small frame) alternators at full capacity for extended periods. However, they were in Full sized pickup trucks, driving at 60mph. So the alternator got lots of cooling, and it isn't clear to me exactly what "stock" means on a modern 3/4 ton diesel pickup.

[Ulstue]

The forums are in german and you need to have a log in. I know this argument, so I am interested in more reports from the practice.

But really, I'm not interested in the discussion of alternator performance at this point, I'm interested in the 12V rated LFPs ability to absorb at a voltage difference of 0.6 - 1.2V from a charger, whatever it is.

Does anyone have a big lab power supply with which he can charge and set constant voltage 14V or 14.2V, and then observe the amps? Would be probably too much for most devices, a current curve of a single cell at 3.5V constant voltage would also be helpfull.

[lmxr]

Originally Posted by Ulstue

But really, I'm not interested in the discussion of alternator performance at this point, I'm interested in the 12V rated LFPs ability to absorb at a voltage difference of 0.6 - 1.2V from a charger, whatever it is.

Does anyone have a big lab power supply with which he can charge and set constant voltage 14V or 14.2V, and then observe the amps? Would be probably too much for most devices, a current curve of a single cell at 3.5V constant voltage would also be helpfull.

Some vendors publish charge curves for their cells. I took a random one from the internet, and that particular one showed that a 1C current lifts the voltage from 3.25 to 3.35 V / cell (for a cell at 10 % SOC). I assume one can linearly approximate this behavior, aka model this as an internal resistance, which eases calculations somewhat. I think this charge resistance is more or less constant over the SOC range. Of course the open circuit voltage of the pack raises with the SOC, up to about 3.375 V/cell for fully charged. For a 12 V pack, multiply the voltages and resistances by four.

I expect you'll have voltage drops in other components as well: the wiring and the diode distributor (if you have one, of course). The wiring resistance can be easily derived from its length (to and from) and size. Voltage drop over a diode distributor is around 0.6 V, somewhat less over a FET-based one. If your BMS uses solid state switches (FETs), then this adds an additional voltage drop.

You can put the sense wire of your alternator on the batteries. That will cover the voltage drop in the positive line. Doing so will increase the charge current, perhaps to undesired levels (assuming a 'dumb' stock regulator). So putting the sense wire closer to the alternator might be preferable in some cases.

[evm1024]

A LiFePO4 bank at less than 100% SOC will charge in CC mode. Or more properly Current Limited mode. And by less than 100% SOC I mean discharged enough for "bulk" charging.

The factors that limit charging current are:

1) Alternators real charging capacity
2) Derating of alternator by temp compensation (in Alt or regulator)
3) Series resistance of charging cabling (might be mitigated by #4)
4) location of voltage sense points (V+ and gnd)

In the case of most if not all inverter/chargers the voltage sense is in the inverter/charger. And thus when charging they sense voltage before the voltage drop in the cables. Higher voltage at the sense points and lower voltage at the battery.

The Balmar MC612 charger has a dedicated voltage sense wire that can be placed on the battery positive terminal and thus will only derate charging by the voltage drop in the negative charging cable.

The Balmar MC614 has a dedicated positive and negative sense inputs and thus when wired with the sense wires on the battery positive and negative terminal will see the actual battery voltage and set charging according.

Any resistance (corrosion etc) in the sense wires will decrease the voltage sensed and reduce the field current and thus the charge current.


At the moment I am doing a capacity test on my LiFePO4 bank. Later today or tomorrow I will use a Victron inverter/charger to recharge the LiFePO4 house bank. Normally I set the charge current to be less that max. But perhaps today I'll set the charge current to max and then see what charging current I get. And what voltage drops there are in the positive and negative charging cables.

[Ulstue]

How big is your battery bank and charger? Which cells? Some Victrons allow you to adjust the charging voltage. If the charger is capable of more than 100A or the battery is not too big, it would be really interesting to know with which charging current the battery bank is charged at 14V charging voltage (not end of charge voltage).

[evm1024]

The bank is made of 700 AH Winston cells. They do not have that capacity as that there were used back when I bought them 7 years ago.

The Multiplus is rated at 120 Amps. And if I recall I have the bulk voltage set for 14.0 volts.

It sounds like you want to know the charge current into the cells when the cells are at 3.5 VPC (14 volts).

At 3.5 VPC depending on the charging current the cells are between 94% and 98% SOC. Normally in this range of SOC the charger has switched from CC to CV (i.e. reached the bulk charge voltage) and the current is decreasing.

Assuming that the bulk charge voltage is set to 3.65 VPC (14.6 volts) I would expect the charge current to remain in CC.

A key point here is that at high charge rates the voltage rise in the upper knee is quite rapid. The change from CC to CV happens when the cells are around 92% SOC and thus in CV the current drops rapidly.

At 92% SOC those 700 ah cells have only about 56 AH to go to be 100% SOC and at 140 amps that happens in about 24 minutes. It would be very easy to overcharge if one set the bulk voltage to something higher than 3.65 VPC in an effort to put the last few percent charge in at high current.

At lower charging rates the voltage rise in the upper knee is more restrained and even then the risk of over charging is high.

I've attached a charging curve from someone else's 90AH cells that shows this point.

I do have a cerbo hooked up to my cells so we will get a full graph of the charging cycle (voltages and currents vs time). Then we can see where the system goes from CC to CV.

I also have a 200 amp alternator that I have derated to about 120 amps. But I do not plan on using that in the next charge. It is also set with a bulk charging voltage of 13.9 volts. It has a MC-612 so it only has positive voltage sense.

[evm1024]

I went and reread you initial post. The short answer is that the sum of the cells internal resistance and the charging cables must be less than 0.00333 ohms in order to charge at high C rates when the battery is nearly full.

For example - With a bulk voltage of 14.0 volts and a bank voltage of 13.6 volts we have a voltage delta of 0.4 volts. Ohms law tells us that 0.4 volts / 120 amps = 0.00333 ohms.

I suspect that in cases where the charging current appears to be limited at low voltage deltas (i.e. at high SOC) that the charging cable resistance is acting to limit the current. This is especially why using heavy charging cables, really good crimps and connections is important. Also, having the voltage sensing point at the actual battery terminals will help compensate for that resistance. The voltage at the charge source will rise until the source current limit is reached or the voltage at the sensing point equals the bulk setpoint.

[Ulstue]

You are all helpful, thank you!

Originally Posted by evm1024

It sounds like you want to know the charge current into the cells when the cells are at 3.5 VPC (14 volts).

Actually, I would be interested to know the maximum charge current that flows when the cells are at 3.2V (12.8V) or 3.3V (13.2V) and the charger is at a constant 14V.

[evm1024]

I'll post a graph in the next day or so when I get the cells charged. It will include the charge current and voltage from low SOC to high SOC. We will be able to see that data.

The charger will not be outputting 14.0 volts (or at least the battery terminals will not be at 14.0 volts). It will be in CC mode with the current at the max that it can supply and the voltage at whatever voltage it takes to get that current.

The charger will be set for 14.0 volts but the voltage will be limited by ohms law.

[donradcliffe]

Originally Posted by evm1024

I suspect that in cases where the charging current appears to be limited at low voltage deltas (i.e. at high SOC) that the charging cable resistance is acting to limit the current. This is especially why using heavy charging cables, really good crimps and connections is important.

Quite the opposite if you are concerned with overloading your alternator. My LFP setup is a 200Ahr battery in a Mercedes Diesel sprinter RV. The stock alternator is 180 amps and with a dumb regulator at 14.1v. The LFP battery is charged by the alternator through a relay which closes when the engine is running.

If the battery is at 50% SOC, I measure 65 amps into the battery from the alternator. I have voltage drops through the relay and probably 25 feet of wire round trip. Guess what? I am very happy with the 65 amps, because it will fully charge my battery in 2 hours of driving, gets nowhere close to the 120 amp limiit of my BMS, and I am not concerned at all with burning out my expensive alternator.

In my boat I never saw more than 70 amps out of the 100 amp alternator with a 400 ahr gel bank, and my alternator never failed in 15 years and 3000 engine hours. For a while I tried to look at voltage drops and get more out of the alternator, but I finally said, "why?"

[kirklkjb]

Cooling of the alternator will have a substantial impact on its performance. What makes the Balmar superior is the cooling and its overall design to run at high output current for extended periods of time. An automotive alternator is designed to run high output for a short period of time to top up the battery after starting. Therefore the cooling of an automotive alternator is optimized for this short duration of high output. Like any design there are tradeoffs. You can enhance an automotive alternator by adding an external fan and external air intake but it will still be designed for an auto use case.