Emergency charging of LiFePo4 with wrong charger?

[Franziska]

Hi,

we installed 1200w of solar panels and 840Ah of LiFePo4 12V batteries this summer.
In almost all situations we should be totally power independant and the solar should be easily able to keep our power topped up.

Now here comes the question, right now we can not afford to add a new charger/inverter and I am wondering if we can not keep/use the old charger which was used to charge the lead acids on rare occassions if we just use it to put some extra amps into the batteries before switching it off manually.

Say the batteries are down to 5% and we need some more power but there is not enough sun and wind. Is anything speaking against manually switching the old Weaco Perfect Charge (1250 = 50A model) on and running it via the genset until we are at about 80% charge and than switch it off manually?
As we are not using it to automatically start charging or stop charging and we are not using float charge, it should not do any damage to the LiFePo4, should it?

One more question, which is kind of similar, if we were to switch the solar panels on at say 20% battery charge and would switch it manually off again at about 80% would we need to worry about any max charging voltages from the solar controller and their cut off voltages?
This one is more of a theoretical question, as in reality we did program some charging voltages in.

[tkeithlu]

The charging voltage for LifePos is about 14v and for lead-acids is about 13.4. Charging efficiency will take a hit and full charge will not be reached. A battery purist may come on and say Don't You Dare. On the other hand, you are just tryiing to get some juice into the system so you can make your coffee and cool the beer, not trying to manage a house battery system. I doubt that it's going to hurt you.

[goboatingnow]

The lead acid charger will be fine just don’t float charge the lithium. It is unlikely to put 100% but lithium’s don’t care if partially charge them

[Franziska]

Originally Posted by goboatingnow

The lead acid charger will be fine just don’t float charge the lithium. It is unlikely to put 100% but lithium’s don’t care if partially charge them

Thanks to both of you. That confirms what I thought.

To my knowledge Lifepo4 actually "prefer" to be used between 20% and 80% of charge rather than charging them to 100%.

Still they tolerate apparently also accidental full discharges or a charge to 100%, as long as this does not become a regular thing.

Never float charge them.

[LoudMusic]

I agree with tkeithlu. So long as the charger is outputting higher voltage than the battery bank is at, you'll be (slowly) charging the battery. I wouldn't do this as a primary means because it's wildly inefficient, but it works.

I don't understand why you want to switch your solar charger off and on. If it's configured correctly it should be monitoring the batteries and apply voltage and current appropriately. You want the solar to bring the batteries to 100% occasionally so that the measured % is correct in your battery monitor.

But if you do intend to use it that way, I'd say yes you would need to worry about it. The measured % on lithium batteries is calculated by measuring the current drawn and restored over time, because the voltage of lithium batteries is very consistent in the middle of charge, unlike other chemistries which have a more consistent voltage slope, so the % is just "what's the voltage". Over time the % of lithium can become incorrect if the values are being measured slightly wrong. Your batteries may have more or less charge than the battery monitor is reporting. Occasionally bringing them to their max rated voltage in order to reset the battery monitor % is important. I try to make sure this happens at least once every couple weeks, but we have so much solar it can actually happen two or three times a week for me. We end up running more loads to bring it back down.

But really just make sure your charger is set correctly

[LoudMusic]

Originally Posted by Franziska

Thanks to both of you. That confirms what I thought.

To my knowledge Lifepo4 actually "prefer" to be used between 20% and 80% of charge rather than charging them to 100%.

Still they tolerate apparently also accidental full discharges or a charge to 100%, as long as this does not become a regular thing.

Never float charge them.

You don't want to hold them at high voltages, but, depending on the manufacturer's specifications, pushing them up to 14.4v or such is perfectly fine and is the only way to reset the battery monitor to actual capacity. After it's reached the equalization voltage the charger should back off. Usually it takes a few minutes for the equipment to decide "we're good here" and switch modes.

[lmxr]

Originally Posted by Franziska

Hi,
[...]
Now here comes the question, right now we can not afford to add a new charger/inverter and I am wondering if we can not keep/use the old charger which was used to charge the lead acids on rare occassions if we just use it to put some extra amps into the batteries before switching it off manually.

Say the batteries are down to 5% and we need some more power but there is not enough sun and wind. Is anything speaking against manually switching the old Weaco Perfect Charge (1250 = 50A model) on and running it via the genset until we are at about 80% charge and than switch it off manually?
As we are not using it to automatically start charging or stop charging and we are not using float charge, it should not do any damage to the LiFePo4, should it?

Having upgraded our batteries we were in a somewhat similar situation as you a few months ago. We reused our old lead-acid charger and it works brilliantly. We have automated the turning on and off though.

From the battery's point of view, there is no difference between a 'lead-acid' charger providing 50 A and a 'lithium' charger providing 50 A. The batteries won't care as long as you stop charging when they have reached full charge, and of course stay within the cell voltage limits etc.

50 A in a 840 Ah bank works out to 50/840 = 0.06 C. You can expect them to reach full charge when the highest cell voltage reaches 3.65 V (derived from https://nordkyndesign.com/charging-m...battery-banks/). Continuing to provide 3.65 V will slowly damage the battery, so charging should be stopped once (or before) this voltage has been reached.

Some older lead-acid chargers burn out when charging lithium batteries because they were not designed to produce their rated current for longer than a quarter hour or because they cannot limit their output current. Therefore you may want to check whether your charger has overtemperature protection. Your Waeco looks like a pretty decent unit so I guess it will do just fine. From the manual I've the impression it can be set to output a steady 13.8 V. 13.8 V is still plenty to charge an LFP battery and will give less issues in case you forget turning it off.

[...]One more question, which is kind of similar, if we were to switch the solar panels on at say 20% battery charge and would switch it manually off again at about 80% would we need to worry about any max charging voltages from the solar controller and their cut off voltages?
This one is more of a theoretical question, as in reality we did program some charging voltages in.

Between 20 and 80 % SOC it is very unlikely that they will be in any other state than bulk, or put differently, they'll not be limiting the charge current or voltage because the battery voltage will be less than the absorption voltage. Therefore I would be very surprised if the charging voltages have any influence. At least under the assumption that you've programmed in an absorption voltage of 13.6 V or more. Be aware that when controlling charging using SOC that most SOC estimators loose accuracy after a few weeks of not reaching a full charge.

[Nico Domino]

Most LiFePo4 have their own BMS and happily charge from most battery chargers. Would not rely on it permanently , but occasionally it's definitely OK.