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DRAFT
Words of Wisdom: “With a few basic design considerations, and an alternator design newer than 25 years old, a direct charging system [for LiFePO4] can be safe, effective, and provide significant benefits. - midwestdrifter1)
There is an often-repeated claim that LiFePO4 house banks will pull monstrous levels of current and destroy alternators if a plain isolator is used instead of a DC-DC charger. This claim is not supported by empirical testing. The data below provide actual measurements to help people make informed decisions. We will see that current demand by LFP banks follows a known formula rather than the opinions of people on the internet.
Short digression: there is a related claim that LiFePO4 cannot be charged fully by isolator. Maybe, but…
So this article will focus on current demand by LiFePO4 house banks.
This won't hurt a bit.
I=V/R. This means “current3) is equal to the difference in voltage4) divided by resistance5)”.
For us, this means there are two factors that dictate what a battery bank (lithium or otherwise) will demand of the alternator:
You don't have to fully understand it or get a I=V/R tattoo, but do keep the basic idea in mind as we plunge ahead.
This section includes installs that one might find in a typical van:
Since it includes screenshots the data will be for installs documented before Sept 15, 2022.
{There is a full list including installs that fall outside the criteria above. It is updated as I find new information – secessus}
The first thing to look at is how much maximum current the banks are actually drawing in relation to their C-rate (capacity rating).
Remember the warnings tell us the C-rate will be 1.0, which is the max the BMS will allow under most circumstances.
The average max current drawn from the alternator is 0.33C, or 1/3rd of what the warnings predict. This the same C-rate that AGM pulls when cycled to 50% DoD.
There is an outlier that pulls 0.67C. That setup has 1awg cables from the chassis to house battery bank, resulting in extremely low resistance (9mR). The practical effect of resistance on current will be addressed below, using that owner's experiments for illustration.
Remember The listed Max current is as a worst-case scenario that is typically only seen when charging first kicks in. With direct-charging current falls off linearly10) as the house bank voltage rises. See below.
A second way to look at this is how much of an alternator's output the bank draw per 100Ah of capacity:
On average, direct-charging 100Ah of LFP consumed ~22% of the alternator's rated output.
There are two failures, and neither of them come from the van-relevant section.
500Ah of LFP direct charged with a 225A alternator. This falls outside the van-relevant window since bank size >300Ah.
Mortons on the Move were able to overheat (not damage) the 225A primary alternator with 500Ah pulling 180A between the tow vehicle and travel trailer. This setup might have worked with the theoretical ~400A capacity of the dual-alternator system, but the second alt was not cutting in the way they expected. They ended up installing a DC-DC charger (30A Orion-TR) to limit current.
300Ah of LFP (no BMS) direct charged with a 90A alternator. This falls outside the van-relevant window since alternator rating <120A.
Victron damaged a 90A car alternator with 300A of lithium on a bench (no airflow as when driving). They were able to charge the same bank from a 100A-120A11) externally-regulated Balmar alternator. The Balmar observes alternator temperature and regulates current to maintain alternator-safe conditions.
Don't direct-charge banks that have 2-3x the capacity of your alternator's rating. Use a DC-DC charger, external regulation, or current reduction with resistance as described elsewhere in this article.
Do not idle to charge.
The owner of the van in the higher-current outlier mentioned above had very low resistance in his wiring. He added resistance to see the practical effect on current drawn by the house bank:12)
We can see that adding resistance makes a noticeable difference in the charge current flowing from alternator to bank.
Thinking back to I=V/R, we should not be surprised that tripling R reduced current to 1/3rd of it's unfettered rate. He provided additional datapoints (table to right).
Although he inserted an actual resistor, any component that increases resistance would have a similar effect:
Let's assume battery resting voltage is 13.0 and alternator voltage is 14.0 and circuit resistance is 20m Ohm.
Let's add 5m Ohm of resistance:
Diode-based isolators incur voltage losses, averaging around 0.5v. This will decrease the difference between battery and apparent alternator voltages, thereby reducing charging current.
It would also reduce final charging voltages, so this might allow vehicles with high chassis voltages like to safely charge LFP. Example: 14.6v → diode-based isolator → 14.1v.
Let's assume battery resting voltage is 13.0 and alternator voltage is 14.0 and circuit resistance is 20m Ohm. Diode isolator cause 0.4v loss. We'll do it with a relay first:
Let's add the diode isolator14) to reduce apparent alternator voltage:
[For this section we will stipulate that alternator voltage is 14.0v and 60A DC-DC's charging setpoint is 14.4v and float is 13.4v. The bank will accept 85A if directly connected to the alternator at low state of charge.]
Charging a 100Ah bank from 20% 100% SoC would take 80Ah. Direct-charging and DC-DC both deliver the same 80Ah over time but in different ways:
This crude and fictional graph illustrates the general patterns of current demand on the alternator from both isolators (purple) and DC-DC chargers (green).
The isolator starts out high at 85A and current starts falling off immediately. It continues dropping gradually as battery bank voltage comes up to 14.0v at about 30 minutes.15) Current continues to drop but voltage is held at 14.0v unless the isolator is disabled.16)
The DC-DC holds a steady 60A until the 14.4v setpoint is reached around 50 minutes. Current drops off steadily until Absorption durations times out at 100 minutes.17). DC-DC drops to float 13.4v and current tapers away.
Note: the crossover point between green and purple at ~20mins marks the time DC-DC charging passes more current than the isolator. By about 35mins the approaches are at breakeven, both having returned about the same Ah to the bank. DC-DC charging voltage might or might not have climbed higher. If one drives for longer periods the advantage may be with DC-DC. If one makes short drives <35mins the isolator might win.
There are good reasons not to attempt direct charging:
If you are still interested, here is one approach to assessing your setup for direct charging lithium:
Caveats:
After you have completed the assessment above here are the required parts: