Connecting Lifepo4 batteries in parallel

[0verk1ll]

Hey all, first time poster. I'm a bit of a n00b so apologies in advance.

TLDR: I currently have a 3ish year old 120ah lifepo4 battery. I want to drop in another battery in in parallel to increase the capacity. My questions are:
1. Is it OK to mix different ages and capacities?
2. Is it a problem if the connection between the two is pretty far (8-10ft)?


Long version:
I've got a camper van where I currently have a diy lifepo4 battery I made using cells from aliexpress and an Overkill Solar BMS. It's about 3 years old and 120ah. It gets charged via about 200w of solar and also the van's alternatore (when I enable that). I am considering getting Starlink and will need more juice to power that. Unfortunately, the stock aux battery location is too small for a larger battery so I thought I'd just buy a drop-in lifepo4 that I could fit elsewhere and wire them in parallel, increasing the capacity with minimal hassle and retaining the battery I already have. However, I'm concerned that maybe it's not OK to hook up batteries of different ages/capacities (I've read mixed things on this). Also, I am curious if the distance between the two batteries would be a problem. The current battery is hooked up to a fuse block that's about 1ft away. The second battery would have to be about 8-10ft away from the current battery and so would also be much farther from the fuse block where the load is coming from. Could this be a potential issue?

Thanks for the help!

 

[Supervstech]

The problem with long distances is the remote battery won't feel the load much...
It won't help the loads but very minimally.
Unless you use HUGE cables and spread the load cables between the banks.

 

[0verk1ll]

Thanks for the response. I figured that the load wouldn't be even but wasn't sure to what degree. Also, I wasn't sure if that even matters all that much. As in, if the smaller/closer battery feels the load more and drains all the way to cutoff, then the load would automatically start getting pulled from the larger/farther battery. Is there any harm in that?

I might also see if I can get some new higher capacity cells that will fit in the current battery's spot to replace it with, but it's a pretty tight space and last I looked I couldn't get over 150ah there.

 

[Supervstech]

It likely won't work that way.
The voltage drop on the distance will cause uneven discharge and charge...
You could mitigate the issue by pulling negative from the far battery, and positive from the large... so, you would need the cables between the batteries to be as large as possible, and the negative or positive cable for the load to be on the far battery, and the other to be on the close battery.
Again, as large a cable as possible.

 

[Ampster]

if the smaller/closer battery feels the load more and drains all the way to cutoff, then the load would automatically start getting pulled from the larger/farther battery.

As mentioned, the distance will create voltage drop which will cause unequal current between the batteries. It is more about the voltage difference between the batteries. I don't think they "feel" the load but that may be another way of explaining the physics of Ohms Law.

 

[0verk1ll]

Got it - thanks for the info. I'm not sure what I'm going to do at this point.

One more, probably dumb question. In something like a vehicle, when running batteries in parallel, is it necessary for the negative terminals of both batteries to be directly connected to each other or can they both just ground on the chassis? That doesn't really affect what I'm trying to do, I'm just curious.

 

[Zwy]

I'd run equal length cables to a central busbar point, then run a larger cable from busbar to inverter. Note I said equal length, both positive cables need to be the same length and both negative cables need to be the same length to the busbars. The positive and negative cables do not need to be the same length.

I would not use a chassis connection as the resistance of the chassis will vary at different points. The idea to keep batteries in balance is to keep circuit resistance the same for both batteries.

 

[Bigbillsd]

It sounds like you might be wasting your money unless you can put them next to one another. And the the old and new connected together might not help much. My SL HP dishy using 2 Kwh per day. 80watts, and 40 watts when in sleep mode. It uses double that if it dealing with a lot of blockage. I just saw those HQST 100ah Lifepos are back in stock around $250 each.

 

[Ampster]

In something like a vehicle, when running batteries in parallel, is it necessary for the negative terminals of both batteries to be directly connected to each other......?

If you are referring to the high voltage traction battery, the industry practice is to not use the chassis and to isolate that from the chassis. I did an EV conversion of a VW ten years ago and even for the small 12volt battery that just ran accessories I preferred to use a separate wire for ground. Part of my reasoning was the chassis was 35 years old and I did not want to have issues. I was already making up a new wiring loom anyway, so an extra wire was not a big deal.

 

[Supervstech]

Got it - thanks for the info. I'm not sure what I'm going to do at this point.

One more, probably dumb question. In something like a vehicle, when running batteries in parallel, is it necessary for the negative terminals of both batteries to be directly connected to each other or can they both just ground on the chassis? That doesn't really affect what I'm trying to do, I'm just curious.

To block is best

 

[0verk1ll]

Thanks for all of the replies. Now that I think about it I could probably just do some sort of switch to switch between the batteries. That way I get the full capacity of both, just at different times. Obviously that means I can only charge one at a time as well but that's a small price to pay for not having to lose the capacity I already have. Still need to do some thinking about this but seems like a decent idea.
Something like this: https://www.amazon.com/Blue-Sea-9001E-Battery-Switch/dp/B001V7TL10?th=1

 

[Zwy]

I don't know why you would install a switch. Serves no purpose. You do need a T fuse between batteries.

This is LFP and it isn't the same as lead acid where the battery ages with sulfation reducing capacity rather quickly. In my battery bank, I have paralleled 280Ah batteries with 304Ah batteries. This is done by proper use of busbars and same cable lengths. The batteries stay in balance.

The correct way to install a second battery is like this. https://diysolarforum.com/threads/connecting-lifepo4-batteries-in-parallel.68770/post-869181 This will keep both batteries in balance.

 

[0verk1ll]

So, if I understand correctly, I'd need two bus bars, one for positive and one for negative. Same cable lengths for positive and negative from each battery to both bus bars. Then positive and negative from bus bar to fuse block and ground can be any length. Correct?

I guess I was just thinking a switch because I don't have a good central location for the bus bar and space around the original battery and fuse block is tight. Not sure how that would affect my solar charging with the added cable lengths but I can't do anything about that without relocating everything.

 

[0verk1ll]

Actually I have one more question, just related to having batteries of different capacities in general. I know having the batteries balanced is important, so is there going to be any problem with the balance if one battery goes offline and then comes back on later? Or will it just balance out over time? I'm not just thinking of when the lower capacity battery runs out before the larger, but also with the different locations of two batteries the low temp cutoff might happen on one before the other, which would cause unequal charging for however long. Just curious if that could pose a problem.

 

[Zwy]

So, if I understand correctly, I'd need two bus bars, one for positive and one for negative. Same cable lengths for positive and negative from each battery to both bus bars.

The negative and positive do not need to be equal lengths. Only both positives/both negatives. The positives can both be 3 feet in length while the negatives 1 foot in length to the bus bars as an example.

Then positive and negative from bus bar to fuse block and ground can be any length. Correct?

After the bus bar the cables can be any length required.

I guess I was just thinking a switch because I don't have a good central location for the bus bar and space around the original battery and fuse block is tight.

The busbars can be located at any point that is convenient. It is the cable length that is critical, the wire has resistance. You want equal resistance for each cable in order for each battery to be at the same voltage.

Not sure how that would affect my solar charging with the added cable lengths but I can't do anything about that without relocating everything.

It affects solar charging by ensuring each battery is at the same charging voltage which is what you want.

You don't need to relocate anything, you only need to make the positive cables the same length and the negative cables the same length from each battery to the busbar. One battery can be a foot away from the busbar and the other 3 feet but as long as both positive cables are 3 feet in length and both negatives are 3 feet in length, both batteries will remain in balance.

Actually I have one more question, just related to having batteries of different capacities in general. I know having the batteries balanced is important, so is there going to be any problem with the balance if one battery goes offline and then comes back on later? Or will it just balance out over time?

The batteries will balance when in parallel if resistance from the busbar to the batteries is the same.

I'm not just thinking of when the lower capacity battery runs out before the larger,

That is not how it works if you have equal circuit resistance for the battery circuits to the busbar. The batteries in parallel just add the capacity of each battery together. Let's say you have a 100Ah battery and a 200Ah battery in parallel. If circuit resistance from each battery to the busbar is the same, the total battery capacity is 300Ah. The 100Ah will not "run out" before the 200Ah. Both will show the same voltage.

Another advantage of batteries in parallel is that is allows for larger loads to be split between batteries. For example, 2 100Ah batteries that have a BMS discharge limit of 100A (1C). Individually, that is all that can be pulled. But with 2 batteries in parallel, 200A can be pulled because each BMS is rated at 100A. You do need to size the cable from busbar to inverter accordingly if you plan on pulling max amps from the parallel batteries.

but also with the different locations of two batteries the low temp cutoff might happen on one before the other, which would cause unequal charging for however long. Just curious if that could pose a problem.

That can be mitigated.

 

[Tomthumb62]

What are the physical dimensions you have to work with in your space for a battery? A DIY battery will take up far less space. You can fit a 200Ah 12v in most battery compartments.

 

[0verk1ll]

The negative and positive do not need to be equal lengths. Only both positives/both negatives. The positives can both be 3 feet in length while the negatives 1 foot in length to the bus bars as an example.


After the bus bar the cables can be any length required.



The busbars can be located at any point that is convenient. It is the cable length that is critical, the wire has resistance. You want equal resistance for each cable in order for each battery to be at the same voltage.


It affects solar charging by ensuring each battery is at the same charging voltage which is what you want.

You don't need to relocate anything, you only need to make the positive cables the same length and the negative cables the same length from each battery to the busbar. One battery can be a foot away from the busbar and the other 3 feet but as long as both positive cables are 3 feet in length and both negatives are 3 feet in length, both batteries will remain in balance.



The batteries will balance when in parallel if resistance from the busbar to the batteries is the same.



That is not how it works if you have equal circuit resistance for the battery circuits to the busbar. The batteries in parallel just add the capacity of each battery together. Let's say you have a 100Ah battery and a 200Ah battery in parallel. If circuit resistance from each battery to the busbar is the same, the total battery capacity is 300Ah. The 100Ah will not "run out" before the 200Ah. Both will show the same voltage.

Another advantage of batteries in parallel is that is allows for larger loads to be split between batteries. For example, 2 100Ah batteries that have a BMS discharge limit of 100A (1C). Individually, that is all that can be pulled. But with 2 batteries in parallel, 200A can be pulled because each BMS is rated at 100A. You do need to size the cable from busbar to inverter accordingly if you plan on pulling max amps from the parallel batteries.

That can be mitigated.

Thanks for the explanations, that really helps and all makes sense. I may just need to get a heating pad for the smaller battery, since it’s closer to the exterior wall of the van.

To answer the question about dimensions, I don’t have the exact measurement handy but it’s about 6.75 x 6.75 x 11.75. The main problem I find with diy cells is the height. Anything over 150ah is over 8 inches high. I do have another alternative location for a battery if I do DIY (larger drop in batteries wouldn’t fit) but haven’t found cells yet that would get here in a reasonable shipping timeframe at a reasonable price. Granted, I have more searching/research to do on that but am initially thinking I’ll get the 230ah Li Time battery while they’re having this sale.

 

[Surferdudemi]

The negative and positive do not need to be equal lengths. Only both positives/both negatives. The positives can both be 3 feet in length while the negatives 1 foot in length to the bus bars as an example.


After the bus bar the cables can be any length required.



The busbars can be located at any point that is convenient. It is the cable length that is critical, the wire has resistance. You want equal resistance for each cable in order for each battery to be at the same voltage.


It affects solar charging by ensuring each battery is at the same charging voltage which is what you want.

You don't need to relocate anything, you only need to make the positive cables the same length and the negative cables the same length from each battery to the busbar. One battery can be a foot away from the busbar and the other 3 feet but as long as both positive cables are 3 feet in length and both negatives are 3 feet in length, both batteries will remain in balance.



The batteries will balance when in parallel if resistance from the busbar to the batteries is the same.



That is not how it works if you have equal circuit resistance for the battery circuits to the busbar. The batteries in parallel just add the capacity of each battery together. Let's say you have a 100Ah battery and a 200Ah battery in parallel. If circuit resistance from each battery to the busbar is the same, the total battery capacity is 300Ah. The 100Ah will not "run out" before the 200Ah. Both will show the same voltage.

Another advantage of batteries in parallel is that is allows for larger loads to be split between batteries. For example, 2 100Ah batteries that have a BMS discharge limit of 100A (1C). Individually, that is all that can be pulled. But with 2 batteries in parallel, 200A can be pulled because each BMS is rated at 100A. You do need to size the cable from busbar to inverter accordingly if you plan on pulling max amps from the parallel batteries.

That can be mitigated.

Was just looking for info related to connecting an additional pack to an existing pack in parallel on the forum. This seems like the situation presented here with:
"... is there going to be any problem with the balance if one battery goes offline and then comes back on later? Or will it just balance out over time?"
to which the answer was:
"The batteries will balance when in parallel if resistance from the busbar to the batteries is the same."

My concern is that there will be a huge surge in current from a bank with higher resting voltage to the bank with lower resting voltage. This can be mitigated by having the packs very close in voltage, but it seems like there would still be a potentially large surge when initally connected, possibly blowing the fuse in one or both of the packs. Is there a "best" way to avoid this?

 

[Zwy]

Was just looking for info related to connecting an additional pack to an existing pack in parallel on the forum. This seems like the situation presented here with:
"... is there going to be any problem with the balance if one battery goes offline and then comes back on later? Or will it just balance out over time?"
to which the answer was:
"The batteries will balance when in parallel if resistance from the busbar to the batteries is the same."

My concern is that there will be a huge surge in current from a bank with higher resting voltage to the bank with lower resting voltage. This can be mitigated by having the packs very close in voltage, but it seems like there would still be a potentially large surge when initally connected, possibly blowing the fuse in one or both of the packs. Is there a "best" way to avoid this?

Best practice is fully charge both to your bulk charging voltage before connecting the batteries, at least into the knee area and over 90% charge. You do not need to get to 3.65V per cell. When I put all 4 batteries into my battery bank, I fully charged each battery before connecting them on a main busbar. If both are close to the same voltage, there won't be enough current flowing to trip a breaker or fuse. Preferred is fully charge both to your bulk charging voltage, this helps keep balance between the batteries from the start.