Can I parallel different ah lifepo4

[pyrofx]

That's what I was afraid of.

[john61ct]

Originally Posted by thomm225

To simplify, the lower capacity battery will charge up first and the charger will see this and shutdown.

The larger capacity battery(s) gets left behind and doesn't get a full charge.

That is not in line with CptPat's report

[thomm225]

Originally Posted by john61ct

That is not in line with CptPat's report

Not sure what Cpt Pat's Report has to do with anything.

I'm just letting the OP know what will happen if he mixes these batteries in parallel as presented in the opening post.

I'm basing this on my knowledge plus experiences with batteries on my boat(s).

How have the batteries on your boats done with dissimilar batteries in parallel.

[newhaul]

Originally Posted by thomm225

To simplify, the lower capacity battery will charge up first and the charger will see this and shutdown.

The larger capacity battery(s) gets left behind and doesn't get a full charge.

huh not accurate. The batteries will equalize to the same (relative) voltage and both Lfp batteries will reach the percent charge point at the same time .

[newhaul]

Originally Posted by thomm225

Not sure what Cpt Pat's Report has to do with anything.

I'm just letting the OP know what will happen if he mixes these batteries in parallel as presented in the opening post.

I'm basing this on my knowledge plus experiences with batteries on my boat(s).

How have the batteries on your boats done with dissimilar batteries in parallel.

the batteries are not dissimilar they are the same chemical makeup.
If the batteries on your boat with differing capacities but same chemistry , are giving you any issues you have them miss wired in paralleling

[mbartosch]

Originally Posted by newhaul

huh not accurate. The batteries will equalize to the same (relative) voltage and both Lfp batteries will reach the percent charge point at the same time .

I agree with this. Assuming identical Lithium chemistry and preferably identical brand but different cell size, connecting different capacity packs in parallel will yield charge and discharge current proportionally to their size. Over time both banks will even out their SoC level.

I can confirm this from my own experience. On Entropy we have a 400 Ah Winston LiFePO4 house bank and two independent portable 40 Ah Winston LiFePO4 battery packs which can be connected in parallel to the house bank and/or solar charger via a 1-2-both-off switch on the charge bus.

We regularly disconnect the mobile packs from the house bank in order to use them for dinghy propulsion and reconnect later to recharge the mobile pack from the solar charger or to extend the house capacity on prolonged cruises.

House bank and both mobile banks have distinct BMS and BMV700 battery monitors.

What I am observing is:

- Reconnecting a mobile pack to the house bank at different SoC will yield a current flowing between both banks. Typically the current I observe is in the area of 5 - 10 A (e. g. mobile pack is at 30 %, house is at 70 %).

- Drawing power from freshly paralleled packs sitting at different SoC will yield a proportionally higher current coming from the higher SoC bank. Conversely, charging the paralleled packs will yield a higher current into the lower SoC pack. This is maintained until both packs roughly are at the same SoC level.

If I leave the mobile pack connected to the house bank during a longer cruise (multiple shallow charge/discharge cycles) I observe the the SoC counters of the mobile packs are typically less than 10 % apart from the SoC counter of the house bank.

[Cpt Pat]

Originally Posted by mbartosch

What I am observing is:

- Reconnecting a mobile pack to the house bank at different SoC will yield a current flowing between both banks. Typically the current I observe is in the area of 5 - 10 A (e. g. mobile pack is at 30 %, house is at 70 %).

- Drawing power from freshly paralleled packs sitting at different SoC will yield a proportionally higher current coming from the higher SoC bank. Conversely, charging the paralleled packs will yield a higher current into the lower SoC pack. This is maintained until both packs roughly are at the same SoC level.

If I leave the mobile pack connected to the house bank during a longer cruise (multiple shallow charge/discharge cycles) I observe the the SoC counters of the mobile packs are typically less than 10 % apart from the SoC counter of the house bank.

Wouldn't you see the same effect with lead-acid batteries? The effect (circulating currents between the batteries of differing SoC) may only be proportionally lower with lead-acids because of their higher effective series resistance.


I chose a threshold voltage or 12.7 volts for removing/restoring my LFE from/to the bus because an LFE will draw practically no charge current at 12.7 volts. It doesn't draw any substantial charge current until the terminal voltage reaches 13.4 volts. So, the lead-acids don't supply current to the LFE at 12.7 volts.


I would never connect a fully discharged LFE to a fully charged lead-acid with a terminal voltage over 13.4 volts. That's when my battery terminal fuses would blow.

[newhaul]

Originally Posted by Cpt Pat

Wouldn't you see the same effect with lead-acid batteries? The effect (circulating currents between the batteries of differing SoC) may only be proportionally lower with lead-acids because of their higher effective series resistance.


I chose a threshold voltage or 12.7 volts for removing/restoring my LFE from/to the bus because an LFE will draw practically no charge current at 12.7 volts. It doesn't draw any substantial charge current until the terminal voltage reaches 13.4 volts. So, the lead-acids don't supply current to the LFE at 12.7 volts.


I would never connect a fully discharged LFE to a fully charged lead-acid with a terminal voltage over 13.4 volts. That's when my battery terminal fuses would blow.

yes that would be the same regardless of battery chemistry as long as all batteries concerned are of the same chemistry.
IE: all Lfp batteries or all Fla batteries or all agm etc.

[Cpt Pat]

Cpt Pat,
I am sorry, I have not followed your boat's configuration.
Do you have a BMS?

My only BMS is the connect/disconnect circuitry I have already described.

What LFP?

Stark Power: http://www.cruisersforum.com/forums/f14/stark-power-lithium-battery-experience-193312.html. The Stark battery has an internal BMS, but its limits are too extreme for my tastes. It DOES NOT have cell balancing, and its BMS voltages are too near the "knees" for me. I treat its internal BMS as only a last-ditch backup.

Are you using SolarPV primarily and not an Alternator? The reason I ask is the 12v 30a relay.
What size is your Alternator?

I have no alternator, nor an engine with an electric starter. My charge sources are: 1) a 30 amp battery charger, 2) 330 watts of solar panels, 3) a towed hydrogenerator. None of those sources can produce more that 30 amps. My maximum discharge current is 15 amps.

Do you wire the APO3 (LFP drop below 13.2v) and BMV-712 (LFP rise above 13.8v) in series so if either one breaks the relay disconnect the LFO?

Yes, exactly. Except, the APO3 is set to 12.7 volts. It supplies the relay coil current, and the switch in the BMV-702 is in series. The BVM-702 relay is configured in "inverted" mode (config setting 11). NOTE: I mistyped the model number earlier: I am using a BVM-702.

Do you know if the BMV-712 can also be used for the low limit? -apparently not, it is a setting but it only has one set of contacts?

It can, but I'm using only the APO3 for low voltage cut-off.

Why doesn't Victron expand the BMV-712 to provide two sets of contacts for the LFP, one for High and one for Low?

That'd be nice.

I assume your Solar MPPT and Alternator Regulator are protecting the LA battery.

Correct. I'm using an MPPT controller, and it's maximum output voltage is 14.4 volts.

What are the potential problems with this approach as compared to having separate discharge and charge buses?

It's simpler but less flexible. The only action I need to take manually is switching off the LFP when the boat is sitting at the dock. I avoid the complexity of needing separate charge control electronics with different charging algorithms. The Victron monitor is added complexity, but I'd want that monitor anyway to keep tabs on the LFP battery.

[thomm225]

Originally Posted by newhaul

the batteries are not dissimilar they are the same chemical makeup.
If the batteries on your boat with differing capacities but same chemistry , are giving you any issues you have them miss wired in paralleling

It's pretty hard to miswire parallel batteries........

The point was that the battery with the smaller capacity will charge up first. The charger or controller will see this and shut down or go to float.

The larger capacity battery then gets left out and doesn't receive a full charge.

This doesn't mean the larger capacity battery will fail immediately but it will fail before the smaller battery

[newhaul]

Originally Posted by thomm225

The point was that the battery with the smaller capacity will charge up first. The charger or controller will see this and shut down or go to float.

The larger capacity battery then gets left out and doesn't receive a full charge.

This doesn't mean the larger capacity battery will fail immediately but it will fail before the smaller battery

your statement is incorrect.
The charger doesn't care about ah capacity it only cares about voltages .

[thomm225]

Originally Posted by newhaul

your statement is incorrect.
The charger doesn't care about ah capacity it only cares about voltages .

Yep, and it will see the smaller battery charge up first and shut down.

If you need details. The charger will see when the voltage on the smaller battery is high enough then shut down.

Basically the larger capacity batteries will fail first if connected in parallel with smaller capacity batteries.

This may take years though so whether to do it or not is up to the guy buying the batteries. It will work. It's just not the best way to go about it

[newhaul]

Originally Posted by thomm225

Yep, and it will see the smaller battery charge up first and shut down.

If you need details. The charger will see when the voltage on the smaller battery is high enough then shut down.

Basically the larger capacity batteries will fail first if connected in parallel with smaller capacity batteries.

This may take years though so whether to do it or not is up to the guy buying the batteries. It will work. It's just not the best way to go about it

both batteries will charge at the same rate to the same cutoff voltage at the same time.

[mbartosch]

Originally Posted by thomm225

Yep, and it will see the smaller battery charge up first and shut down.

If you need details. The charger will see when the voltage on the smaller battery is high enough then shut down.

As Newhaul wrote this is not correct. Two paralleled batteries form an electric mesh with two nodes (common plus and common minus). These two nodes are connected to the charger. Assuming ideal conductors (i. e. zero resistance) the potential at both batteries' poles will always be identical, and the charger will "see" one single "resulting" battery.

Now in the real world there are no ideal conductors, hence the potential at the battery poles will be different, and the potential will also vary with the current running through the individual batteries. Now, if the battery capacity values are not too different and the wires are of similar length and identical cross section the difference is usually negligible.

This means that properly wired, paralleled batteries with identical chemistry will result in the same SoC for each battery.

[newhaul]

Originally Posted by mbartosch

As Newhaul wrote this is not correct. Two paralleled batteries form an electric mesh with two nodes (common plus and common minus). These two nodes are connected to the charger. Assuming ideal conductors (i. e. zero resistance) the potential at both batteries' poles will always be identical, and the charger will "see" one single "resulting" battery.

Now in the real world there are no ideal conductors, hence the potential at the battery poles will be different, and the potential will also vary with the current running through the individual batteries. Now, if the battery capacity values are not too different and the wires are of similar length and identical cross section the difference is usually negligible.

This means that properly wired, paralleled batteries with identical chemistry will result in the same SoC for each battery.

the best way to wire multiple 12v batteries would be pos to pos neg to neg with the pos and negative leads coming from opposite ends of the bank. ( this is for a 12v bank)

ie 2 batteries both positive wired together and both negatives together with the positive for the bus coming off of battery A and the negative comes off of battery B ( when there are two batteries in parallel. )