To start with I have a Xantrex SW3000 inverter/charger. This is an older unit part number 815-3000. It does not have programmable voltages and only has Flooded,
AGM and Gel profiles. There is an OEM Preset which is could be set at a
service center for a large order of inverter/chargers and does not help us at all.
The follow on machine is the SW3012 (815-3012) which does have a user programmable
battery setting which would allow setting voltages that would
work better with
LiFePO4 batteries. Sadly, I do not have that model so I cannot speak about how well it would
work.
Back to the SW3000. First off you really need to have the SCP (system control panel) to control your SW3000 to have any chance of
charging LiFePO4 without significant risk of an expensive failure. And you also need an accurate way of measuring the pack voltage at the pack. I’ve been using a calibrated Fluke meter clipped to the pack terminals. My
Victron BMV-602S which measures voltage at the
Battery disconnect switch (about 5’ of 2/0 cable from the pack) shows a voltage drop in that 2/0 cable of over 16 mV at high charge rates. (I need to move it closer to the pack).
In real world terms it does not matter which battery type you select. The absorption voltage is 14.4 (14.3 AGM) volt for all battery types and is too high for LiFePO4 in my mind. And the SW3000 is capable of charging at 150 amps. In my setup I see a max charge rate of around 135 amps when the pack is 50% discharged. At a 14.4 charge voltage and up to 150 amps you cannot turn your back on it. The upper knee is quite abrupt at this voltage and
current.
The
charger runs in constant
current mode for all phases while charging a LiFePO4 bank. What to do? Well of course the answer is to dump the SW3000 and get another charging solution. In the meantime here is what I do.
First, remember that this
charger cannot be allowed to charge unattended so don’t even think of letting it have its way with your house bank. It is only for charging your bank at the
dock or from a
generator under adult supervision.
My first
rule is that the charger is disconnected from the battery (at the inverter/charger isolation switch) when not being used. When disconnected from the battery the SCP will not be energized.
I set the battery type to
AGM with a 14.3 volt absorption voltage, and select 2 stage charging. Also, there is a parameter “Force Charge” that you should set to off. If this is on (which is the default) anytime the unit sees AC it will automatically turn on the charger. You want to manually turn on the charger.
The
single most important parameter that we can adjust in the SW3000 is the “Max Chg Rate” parameter. This parameter allows us to set the maximum current that the charger will supply. The parameter is defined as Battery AH setting / 5 * Max Chg Rate. So in the case of my bank I’ve set the battery capacity parameter to 700 and with Max Chg Rate set to 75% we get 700/5*.75 = 105 amps. With a max Chg Rate of 12% we get 700/5*.12 = 16.8 amps.
Initially my pack was top balanced and fully charged on the bench then installed in the
boat. I consider a pack voltage of 13.9 volts and a charge rate of less than 20 amps to be fully charged. My
Victron battery
monitor was manually set to 100% SOC at that time. At the
dock I typically have all charging sources disconnected.
I do have to resist the temptation to keep the pack fully charged. This is something left over from years of lead acid…. Using the battery
monitor I tend to charge the bank when it gets down to a 75% or 60% SOC. Still pretty high but it does reduce the charge time.
Remember this is at the dock but it could be using a
generator as well. First I hook up my Fluke to the pack. Moving the battery monitor may give me good results. I’ll report back on that later once I have more info. Then I turn on the battery to
inverter switch and wait for the SCP to boot up. Once all is ready I set the max charge rate to 100% and enable the charger. The battery monitor shows 130 amps. I keep a close eye on the pack voltage and when it reaches 13.8 volts I change the max charge rate to 50%. The pack voltage drops to 13.65 or so at 70 amps charge. I monitor the pack and when it again reaches 13.8 I set the max charge rate to 14%. At 14% the pack voltage again drops and the charge current is about 20 amps. Continuing to monitor the pack voltage I stop charging when the pack voltage reaches 13.8 volts and call it full. Sometimes I let it go to 13.9 volts. If need be I reset the SOC on the BMV-602S to 100%.
Then I disconnect the charger/inverter from the battery (with the disconnect switch). No chance of the charger coming on and trashing my pack. The AC side is still connected and
shore power is passed through the SW3000’s transfer switch to the
boats AC panel.
When away from the
shore power I turn on the inverter/charger to battery switch when I want to use the
inverter and back off again when I’m done with the inverter.
In short the SW3000 is not to be trusted with your LiFePO4 bank. You can use one to charge your bank but you must monitor it closely. Don’t even step into the head while charging. And swap it out for something more suitable when you get a chance
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