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Topic: LiFePO4 House Battery Charging and the Effect of Path Resistance (Read 266 times) previous topic - next topic
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LiFePO4 House Battery Charging and the Effect of Path Resistance
Any charging circuit for the house batteries includes the internal resistance of the batteries and the resistance introduced by the cabling, grounding paths, circuit breakers or fuses, and connection losses. For this discussion, I will call this Path Resistance (PR). When upgrading to LiFePO4 batteries with built-in battery management and the capability of high-current charging all the way to full, expectations may not be met if PR is high. If you have an accurate battery monitor to measure charge current and an accurate digital voltmeter, you can measure your system PR.

You need to start by discharging the battery bank to a point you can measure a steady large charge current for a period long enough to take the voltage measurements. This would typically mean charging from the converter. The measurements you need are:
Voltage at the converter terminals, positive output (Vcp), Negative output (Vcn), at the battery terminals, positive (Vbp) and negative (Vbn). From the battery monitor, charge current in Amps (A). Plug in the values: PR = (Vcp - Vbp + Vbn - Vcn) / A.

Using the case of 200Ah of LiFePO4 batteries and a charger capable of 50A continuously at up to 14.6V. Note that the charger is current limited, so when the charge current is 50A, the charge voltage will fall to Vbp + (50 X PR). I will show the cases for PR of respectively 0.01 Ohms, 0.03 Ohms, and 0.05 Ohms, typical of hookups that range from excellent to very good. The lithium batteries will be starting from a 30% state of charge of 13.0V resting, so down by 120 Ahrs, and ending at a full charge of 13.6 V resting. Charge current is calculated by (charger voltage - battery voltage) / PR. If this is higher than 50 Amps, then 50 Amps is used.

The case where PR = 0.01 Ohm: Initial charge current (14.6 -13.0) / 0.01 = 160 Amps. Thus, we use 50 Amps, and the calculation 50 X 0.01 = 0.5 V gives the voltage drop between charger and battery. Thus initially, the charger voltage drops to 13.5 V. When the battery has charged to 13.6 V, the charger voltage will be 14.1 V, still less than 14.6V, so the charger can supply 50 Amps though the entire process. Charge time is thus 120 / 50 = 2.4 hrs.

The case where PR = 0.03 Ohm: Initial charge current (14.6 -13.0) / 0.03 = 53.3 Amps. Thus, we use 50 Amps, and the calculation 50 X 0.03 = 1.5 V gives the voltage drop between charger and battery. Thus initially, the charger voltage drops to 14.5 V. When the battery has charged to 13.6 V, the charger voltage will be 14.6 V, its voltage limit, so the charger will supply (14.6 - 13.6) / 0.03 = 33.3 Amps. For simplicity, averaging the beginning and ending charge currents we get an average charge of ~42 Amps. Charge time is thus 120 / 42 = 2.9 hrs, still good.

The case where PR = 0.05 Ohm, which might be the case for many older setups - some deterioration of the connections and in the circuit breaker. The voltage drop at 50 Amps would be 50 X 0.05 = 2.5 Volts, which is more than charging voltage differential: 14.6 - 13.0 = 1.6 Volts! So, calculating the maximum charge current, we get 1.6 / 0.05 = 32 Amps at the start of charging. When the battery reaches 13.6 Volts, we have (14.6 - 13.6) / 0.05 = 20 Amps. So, taking the average charge current, we get 26 amps. So, our time to full charge would be 120 / 26 = 4.6 hrs.

The salient point here, with LiFePO4 batteries, is slight differences in the quality of the cabling can make dramatic differences in performance. A change in the PR from 0.01 Ohm to 0.03 Ohm to 0.05 Ohm changed charging times from 2.4 to 2.9 to 4.6 hrs to full charge. Note that lead-acid battery chemistry has a much higher internal resistance than lithium and different voltages at different states of charge and temperature, so this analysis is to simplified to be useful there.
2004 FL
2013 Honda Fit
 
Re: LiFePO4 House Battery Charging and the Effect of Path Resistance
Reply #1 –
Nice explanation of how voltage drop affects charging time and efficiency.
Cable size can dramatically affect resistance over the total run from the converter or solar controller.
Below is a wire chart showing what size wire should be used for various loads and the total distance of the wire run.
I use the 3% chart to select the wire size and then upsize to the next wire gauge in an effort to lower the voltage drop to near 1%.
In battery charging systems, small voltage drops can significantly waste charging power and lead to longer share times.
Marine Wire Size and Ampacity | West Marine

Crimp-on lugs and connectors can increase in resistance if they are not sealed, and resistance-increasing corrosion occurs.
Learning how to use a VOM (volt-ohm meter) to periodically check the voltage drop in your rig's charging systems can reap rewards when poor connections or undersized wires are found and repaired.

Larry
Larry
2003 23.5' Front Lounge, since new.  Previously 1983 22' Front Lounge.
Tow vehicles  2020 Jeep Wrangler Rubicon, 2001 Jeep Cherokee
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