Parallel or buss bar

[Bob S]

With the price of LifePo4 for Black Friday it’s a no brainer. My GC batteries will be going into their 8th season. I’m looking at two 300ah batteries and am not sure if it is better to parallel them and use 1 300amp T fuse or run each battery separately to a pair of buss bars. Im in the process of creating a wiring diagram.

 

[dLj]

With the price of LifePo4 for Black Friday it’s a no brainer. My GC batteries will be going into their 8th season. I’m looking at two 300ah batteries and am not sure if it is better to parallel them and use 1 300amp T fuse or run each battery separately to a pair of buss bars. Im in the process of creating a wiring diagram.

You have to make sure that the wire lengths for each battery are exactly the same length - both positive and negative wires.

dj

 

[dlochner]

Somewhere on the Victron website there is a good manual on wiring. In it they offer a several ways of wring batteries in parallel. The current thinking is using busbars does a better job of evenly distributing loads and charging currents across all batteries. Regardless of the method used, it is important to have all cable lengths equal.

Victron also has the Lynx Distribution system, not sure if it is appropriate for your application. I believe the latest model incorporates Class T fuses instead of Mega fuses.

Wiring Unlimited

www.victronenergy.com

 

[Johann]

With just two batteries you can’t improve on the diagonal wiring method in terms of electrical resistance. You do need to make sure the interconnecting cables are the same length (the short positive and negative connecting the two batteries).

If you choose to run each battery separately to positive and negative busbars, then standard practice is to make the two positives equal and the two negatives equal, but not necessarily the positives equal to the negatives.

In reality, whichever method you choose you will likely see differences in charge and discharge rates simply due to the different internal resistances in each battery. That is something you should check for after installation, and if the difference is significant try to determine why. If it is a bad connection or cable you can fix it, but if it is just the difference in batteries there isn’t much you can do.

 

[dLj]

In reality, whichever method you choose you will likely see differences in charge and discharge rates simply due to the different internal resistances in each battery. That is something you should check for after installation, and if the difference is significant try to determine why. If it is a bad connection or cable you can fix it, but if it is just the difference in batteries there isn’t much you can do.

How do you check for that?

dj

 

[Johann]

How do you check for that?

dj

Charge to 100% individually. Then connect the batteries in parallel and charge again to absorption voltage and keep that voltage until then current going into each is <1% Ah capacity and roughly equal. Then put on a moderate-heavy load and check the output from each battery. Re-check every few minutes for 30 minutes or so. Also during this check look for any hotspots. If the batteries have BT BMS I would still check with a clamp meter to verify the BMS readings are accurate.

 

[dLj]

Charge to 100% individually. Then connect the batteries in parallel and charge again to absorption voltage and keep that voltage until then current going into each is <1% Ah capacity and roughly equal. Then put on a moderate-heavy load and check the output from each battery. Re-check every few minutes for 30 minutes or so. Also during this check look for any hotspots. If the batteries have BT BMS I would still check with a clamp meter to verify the BMS readings are accurate.

This doesn't make sense to me. LiFePo batteries absorb whatever power you throw at them until fully charged. Then they shut down taking more current. It is an inherent difficulty running them and not blowing alternators. Sorry, I'm not seeing how this works in practice.

dj

 

[Johann]

This doesn't make sense to me. LiFePo batteries absorb whatever power you throw at them until fully charged. Then they shut down taking more current. It is an inherent difficulty running them and not blowing alternators. Sorry, I'm not seeing how this works in practice.

dj

So you determine what absorption voltage you will use. Let’s say you set your 80A charger for 14.2V absorption and you battery is 300Ah currently at 50% SOC and 13.2V. As soon as you begin charging with 80A the voltage will jump to around 13.4V. As you approach 90% the voltage slowly rises but the amps accepted it still 80A. Then around 95% or so the voltage rapidly rises to 14.2V and then the amperage accepted begins to decrease slowly. After about 15-30 minutes at 14.2V the amps accepted will be down to less than 3A and you can consider the battery at 100%.

I installed my 12V LFP bank of three 315Ah batteries in 2020 and have just over 100 cycles on them. I have lots of data on charging and discharging.

 

[dLj]

So you determine what absorption voltage you will use. Let’s say you set your 80A charger for 14.2V absorption and you battery is 300Ah currently at 50% SOC and 13.2V. As soon as you begin charging with 80A the voltage will jump to around 13.4V. As you approach 90% the voltage slowly rises but the amps accepted it still 80A. Then around 95% or so the voltage rapidly rises to 14.2V and then the amperage accepted begins to decrease slowly. After about 15-30 minutes at 14.2V the amps accepted will be down to less than 3A and you can consider the battery at 100%.

I installed my 12V LFP bank of three 315Ah batteries in 2020 and have just over 100 cycles on them. I have lots of data on charging and discharging.

So I'm trying to fully understand - in doing so I'll have to also talk about my batteries as we are running fairly differently. I have 4 - 100Ah batteries. I typically charge them at about 200 amps. When they are close to full, that will drop down to about 80 amps, in just a few minutes that will go down to about 15 amps, shortly then to about 8 amps and then they go to 0. I've never seen 3A or less. But maybe it's because of the charge rate I'm using.

I don't have an easy method to charge each battery individually, but I don't understand why you would do that, and then charge them again from full charge state hooked up in parallel. What does that do? I would think in my case, there would not be any charge current being accepted.

Then you say to discharge the batteries with a moderate to heavy load while hooked up in parallel. Check the batteries for hot spots and measure the output from each battery. I don't know that a BT BMS is... but I'd have to do this check with a clamp on amp meter anyway as the way I measure the output is via a common wire on the output side of my parallel battery connection - I have no way to measure the individual battery output current unless I were to put a amp meter on the individual battery cable running to the common bus. so when measuring the current out put from each battery - what would be considered OK and what would not? Like each one within say 5% or something like that?

What is the purpose of charging each individual battery, then charging these fully charged batteries again in parallel?

Can you explain please?

dj

 

[Johann]

So I'm trying to fully understand - in doing so I'll have to also talk about my batteries as we are running fairly differently. I have 4 - 100Ah batteries. I typically charge them at about 200 amps. When they are close to full, that will drop down to about 80 amps, in just a few minutes that will go down to about 15 amps, shortly then to about 8 amps and then they go to 0. I've never seen 3A or less. But maybe it's because of the charge rate I'm using.

That drop to 0 may be the BMS turning off the charge FETs when one of the cells exceeds the voltage limit. That happens often if charging to 14.6V or even 14.4V.

I don't have an easy method to charge each battery individually, but I don't understand why you would do that, and then charge them again from full charge state hooked up in parallel. What does that do? I would think in my case, there would not be any charge current being accepted.

I would just do this before putting the new batteries in parallel… just so there isn’t a possibility of a large current flow between the them when connected. Charging them again once the are paralleled just ensures they are all truly full.

Then you say to discharge the batteries with a moderate to heavy load while hooked up in parallel. Check the batteries for hot spots and measure the output from each battery. I don't know that a BT BMS is... but I'd have to do this check with a clamp on amp meter anyway as the way I measure the output is via a common wire on the output side of my parallel battery connection - I have no way to measure the individual battery output current unless I were to put a amp meter on the individual battery cable running to the common bus. so when measuring the current out put from each battery - what would be considered OK and what would not? Like each one within say 5% or something like that?

I am just checking connections, crimps, etc for hotspots. BT BMS is Bluetooth BMS that will give you voltage, SOC, and amps in/out on an app. As far as difference I would say 5% is good, even 10%. I have 2 batteries that are within 5% but the third is sometimes 20% less than the my best battery. That 20% caused my to do A LOT of investigation…. Swapping cables and battery positions, re-torque etc, but in the end I determined everything was fine it was just higher resistance in that battery. Even with the higher resistance it doesn’t get warmer than the other two. The internal BMS and battery temp sensors of all the batteries are always within 2°F. But if there was a bad connection it should show up as a large difference as well.

One thing of interest I have noticed is there are five different voltage stages of LiFePO4 on discharge. And these stages allow the higher resistance battery to catch up and get back in balance. Remember they are in parallel so the voltages have to be very close. The first stage from 100% down to about 70% the voltage is flat and this is where I see the biggest differences. Then from 70%-65% there is a voltage decrease and as the faster discharging batteries reach this their output decreases and the other battery increases output and catches up to the other two. Then from 65% to about 40% they will spread out again as the voltage is flat in this range. Once down below 40% the voltage start to decrease again and the third battery catches up again. Then the last stage is the lower knee and they all stay together in this.

 

[dLj]

That drop to 0 may be the BMS turning off the charge FETs when one of the cells exceeds the voltage limit. That happens often if charging to 14.6V or even 14.4V.

That's correct.

I would just do this before putting the new batteries in parallel… just so there isn’t a possibility of a large current flow between the them when connected. Charging them again once the are paralleled just ensures they are all truly full.

Ah, I see - I'm well past that. Been living with these for some time now...

I am just checking connections, crimps, etc for hotspots. BT BMS is Bluetooth BMS that will give you voltage, SOC, and amps in/out on an app. As far as difference I would say 5% is good, even 10%. I have 2 batteries that are within 5% but the third is sometimes 20% less than the my best battery. That 20% caused my to do A LOT of investigation…. Swapping cables and battery positions, re-torque etc, but in the end I determined everything was fine it was just higher resistance in that battery. Even with the higher resistance it doesn’t get warmer than the other two. The internal BMS and battery temp sensors of all the batteries are always within 2°F. But if there was a bad connection it should show up as a large difference as well.

Yes, I did all that hot spot inspection at first installation. I've never measured the battery output from the individual batteries.

One thing of interest I have noticed is there are five different voltage stages of LiFePO4 on discharge. And these stages allow the higher resistance battery to catch up and get back in balance. Remember they are in parallel so the voltages have to be very close. The first stage from 100% down to about 70% the voltage is flat and this is where I see the biggest differences. Then from 70%-65% there is a voltage decrease and as the faster discharging batteries reach this their output decreases and the other battery increases output and catches up to the other two. Then from 65% to about 40% they will spread out again as the voltage is flat in this range. Once down below 40% the voltage start to decrease again and the third battery catches up again. Then the last stage is the lower knee and they all stay together in this.

I'm wondering if this kind of testing can be used to determine if older batteries are starting to go.... They are supposed to last a long time, but I am always skeptical.... And would very much like to be able to test especially if one is entertaining heading out for a more extended trip. Maybe do this kind of testing like once a year, keep records, and see if there are changes. What do you think?

dj

 

[Johann]

I think that could work. If you notice a particular battery contributing less over a couple of years you could pull it out and test its capacity individually… maybe catch a problem before the warranty runs out.