Different Type Batteries Connection Risks?

jesfl

Solar Enthusiast
Joined
May 17, 2020
Messages
80
I just spent nearly two hours reading related posts here. I am a limited knowledge beginner. I have learned a lot from this forum. But, I still don't understand one nagging issue in my mind.

A quick recap:
- Early this year I had four (cheap) Chinese 150Ah LiFePo4 batteries fail, i.e. would not charge. With help from this forum, I learned the cause was the cheap built-in BMS.

- To keep my system running, I purchased a new "Lynx" 12V 200Ah LiFePo4 Prismatic battery with a built-in Daly BMS. It has been working great.

- Recently, the Chinese seller of the previously failed batteries sent me a "free" 200 Ah LiFePo4 battery. I assume because of the size it uses the larger round cells -- which are cheaper I am sure -- like the batteries that failed. It is in a case, so I do now know anything about the built-in BMS. I prefer not to tear it apart and replace the BMS. But, I suppose that is one option.

My worry is:

Is there any risk of damage to my new Lynx battery if I wire these two batteries in parallel . . . because of the different LiFePo4 battery types and unknown BMS compatibility? The positive system connection would be from the new Lynx battery and the negative connection from the replacement round-cell battery terminal and unknown BMS.

If this can create problems, is there any solution?

Thank you in advance for your help in clarifying.

jesfl
 

Northernchateau

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Apr 14, 2021
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195
Location
Worth, NY
Being you already had a failure from the one source I would not be using those 2 different batteries in parallel.
 

DThames

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Nov 22, 2019
Messages
1,422
With a BMS in each battery, connecting them in parallel (if connected at the exact same voltage and nearly same state of charge) should not cause any problem. It would also be helpful to install a DC distribution block and fuse each battery as/before you connect it to the DC block. If you fused each battery at 100A you would be fused at something below or close to what each BMS rating is but still combined have plenty of current. If something odd happened your fuse should blow before even the worst case in any one battery would create a problem for the other battery.
 

jesfl

Solar Enthusiast
Joined
May 17, 2020
Messages
80
A life interruption kept me from responding faster. My apology.

I very much appreciate both perspectives about my issue.

To Northernchateau:

You wrote exactly my fear and my reason for asking the question.

For perspective, my choice is (a) either to limp along with only one 200 Ah LiFePO4 battery for my system (which is getting me by, but without being able to use my microwave when on battery/inverter power), or (b) use both mis-matched batteries.

What I'm trying to understand is (1) what is the risk if the "free replacement battery" with the round LiFePO4 cells goes crazy somehow while wired in parallel, and (2) are there any protections I can build in?

Any additional thoughts will be sincerely appreciated.

To DThames:

First, please remember, a real beginner here, I don't really understand what you mean when you say ". . . (if connected at the exact same voltage and nearly same state of charge) should not cause any problem."

Here's what I think/hope you are saying? I can certainly assure both batteries are 100% charged before connecting them. My current/new 200 Ah Lynx battery typically shows 13.85 volts on my Aili shunt-connected voltmeter when it reaches 100% charged/200 Ah available. Then, my EPEVER solar charge controller apparently slow charges for a while and it will show 14.4V to 14.5V before the sun sets and I start using the battery power. The mis-matched, old-style-round-cells 200 Ah battery I have charged via a small lithium-capable charger and shows 14.44 volts via my multimeter, not connected to anything.

So, you are saying I should assure both batteries are charged to the 14.4V to 14.5V level (100%) before connecting. Correct?

Now, I have distribution bars/buss bars exactly like the ones you linked, but I am not using those in my current wiring. Here's the current setup:

-- there is a 200amp fuse connected to the positive terminal of the single 200Ah battery,
-- a 13" 4/0 wire (yes, excessive, I know) from that fuse to a battery on/off switch,
-- a 5" 4/0 from the on/off switch to a 250amp breaker, and
-- an 11" 4/0 wire from the breaker to the positive on my inverter.

I had to upgrade to the 200amp inline fuse at the positive battery terminal because I kept blowing smaller fuses whenever I tried to run the microwave off the inverter. Everything else worked fine via the inverter, but the 1100 watt microwave would cause problems. I still cannot run it off the inverter. I now believe this is an inverter problem, not a wiring or other system problem. I have come to the conclusion I must purchase a lower-voltage inverter (that is returnable) and test it to prove/disprove the inverter is the cause of the numerous blown fuses during my testing?

-- there is no fuse on the negative battery post,
-- there is 18" of 4/0 wire from the negative battery terminal to the AiLi shunt, and
-- almost 22" of 4/0 from the Aili shunt to the negative connection on the inverter.

I've read the pos/neg sides of battery wiring are supposed to be the same length? Obviously, I can easily add 4/0 wire length to the positive side battery connection if that is a critical path issue?

My solar charge controller wires are connected directly to the battery terminals. On the positive side after the fuse at the same connection point as the 4/0 wire to the on/off switch. On the negative side directly to the negative battery post.

The way I was told to wire multiple batteries in parallel was with the positive system connection to the positive post on the first battery in parallel, and the negative system connection to the negative post on the last battery in the parallel set. Is that not correct?

If I understand correctly, you are suggesting both batteries be connected to the distribution/buss bar's pos/neg with fuses on the positive terminal of both batteries? Correct? That's instead of the above parallel wiring method? I was actually told by the inverter manufacturer not to wire through distribution/buss bars because of the current loss from the additional connection(s).

But, if the distribution/buss bars wiring scheme provides a bit of additional protection in case the old-style battery goes into a destructive mode, I certainly will make that wiring change.

I have no idea the brand, capability, etc. of the built-in BMS on the old-style-round LiFePO4. That's the part that worries me most, given the BMS failures (3) in my previous batteries.

Any follow-on thoughts are most welcomed.

And, again, thank you both very much for your expertise.

jesfl
 

DThames

Photon Sorcerer
Joined
Nov 22, 2019
Messages
1,422
Regarding making the connection when putting batteries in parallel. Yes, both fully charged is correct (same state of charge). You would like to see the same voltage as well. What I have done was charge battery #1. Charge battery #2. Measure (again) the voltage on battery #1 and record the value. Because battery #2 was charged last, it will most likely have a slightly higher voltage. Put battery #2 under a light load and monitor its voltage. When #2 gets down to #1's voltage, connect them together in parallel. This level of detail is not really necessary but avoids any risk of a large current flow as the battery setting to the same voltage after connection.

About cabling parallel batteries. What you were told is correct in the sense that parallel is positive to positive and series is positive to negative. Suppose you have 10 batteries that you want to put into parallel. New concerns pop up. How big should the cable be between 1 and 2, between 3 and 4, and so on......because the current that flows in the cables between the batteries add up as you go down the row. Another, where do you connect to take away power? Do you connect at the end or in the middle, on in more than one place? The ideal would be all batteries connected to a common distribution point. Yes with the same length cable is ideal, IF the batteries are exactly the same. The cable will have a tiny resistance. The batteries will also have a tiny internal resistance. The batteries can also have different capacities (your case and also my case). The worry with same size and length cable produces more meaningful results when everything is exactly the same with the batteries. Otherwise, you need to be sure that the cable is large enough gauge to carry the load with very little lose. Just FYI, I have 2 old, well used 200 ah batteries and one new 280 ah. The two old ones are tied cell to cell and use one BMS but each has their own cables to a large terminal block. The new 280 has its own BMS and is connected to the same block. There are 2 inverters connected to give me split phase 120v/240v. I also have an old inverter still connected (normally off) as well. It is true that more connections and more cable length are conditions that are not desired but some number of connections and come length of cable is needed. I wouldn't cut longer cable and coil it up somewhere (to make them the same length) unless I was dealing with trying to feed antenna data into a radar or something where timing from a set of sensors was critical.

Anyway, that is my opinion.
 

jesfl

Solar Enthusiast
Joined
May 17, 2020
Messages
80
Regarding making the connection when putting batteries in parallel. Yes, both fully charged is correct (same state of charge). You would like to see the same voltage as well. What I have done was charge battery #1. Charge battery #2. Measure (again) the voltage on battery #1 and record the value. Because battery #2 was charged last, it will most likely have a slightly higher voltage. Put battery #2 under a light load and monitor its voltage. When #2 gets down to #1's voltage, connect them together in parallel. This level of detail is not really necessary but avoids any risk of a large current flow as the battery setting to the same voltage after connection.

About cabling parallel batteries. What you were told is correct in the sense that parallel is positive to positive and series is positive to negative. Suppose you have 10 batteries that you want to put into parallel. New concerns pop up. How big should the cable be between 1 and 2, between 3 and 4, and so on......because the current that flows in the cables between the batteries add up as you go down the row. Another, where do you connect to take away power? Do you connect at the end or in the middle, on in more than one place? The ideal would be all batteries connected to a common distribution point. Yes with the same length cable is ideal, IF the batteries are exactly the same. The cable will have a tiny resistance. The batteries will also have a tiny internal resistance. The batteries can also have different capacities (your case and also my case). The worry with same size and length cable produces more meaningful results when everything is exactly the same with the batteries. Otherwise, you need to be sure that the cable is large enough gauge to carry the load with very little lose. Just FYI, I have 2 old, well used 200 ah batteries and one new 280 ah. The two old ones are tied cell to cell and use one BMS but each has their own cables to a large terminal block. The new 280 has its own BMS and is connected to the same block. There are 2 inverters connected to give me split phase 120v/240v. I also have an old inverter still connected (normally off) as well. It is true that more connections and more cable length are conditions that are not desired but some number of connections and come length of cable is needed. I wouldn't cut longer cable and coil it up somewhere (to make them the same length) unless I was dealing with trying to feed antenna data into a radar or something where timing from a set of sensors was critical.

Anyway, that is my opinion.

Thank you, thank you. That explanation is VERY helpful. I appreciate your time. I am seeing the picture much more clearly. It appears I need to find some larger distribution blocks, as mine are only 4 posts.

I will tackle the rewiring and additional battery connection this weekend.

And, never in my lifetime will I be attempting to connect an antenna to radar. Waaaay beyond beginner blundering.

Again, your opinion is highly valued.

jesfl
 

DThames

Photon Sorcerer
Joined
Nov 22, 2019
Messages
1,422
On my terminal block I have 2 lugs on some posts. Not sure what best practice is but if connection is good, I don't see a problem.
 
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