Adding a third drop in LiFePo4 to two older ones in parallel

Edge

New Member
I have two Renogy 170 ah batteries in parallel on my boat. They are about three years old and according to my Rover charge controller they have been fully charged 116 times. I would like to purchase a third Renogy 170ah and add it to the existing 2 batteries in parallel. Any advice? What will be the pitfalls of doing this? advice? What will be the pitfalls of doing this? Note these are not the newer Renogy smart batteries.
 

Roswell Bob

Solar Addict
I think you want to make sure resting voltage (between strings) is close when you make the connection. You want to consider fuse arrangement in case you have a faulty cell. In other words - you probably want to put a fuse in each string. There is an incredible amount of energy in these things and you don't want it taking off on you. Place I worked at burned to the ground when one went haywire.
 

Edge

New Member
Thanks for your input Solar Addict. Each of these batteries has its own BMS and internal breaker, so I'm not sure why I need to put fuses between them
 

time2roll

Photon Sorcerer
I don't think you will have any issues with adding one or two more newer batteries in parallel with the existing pair.
Any issues are more of a lead-acid thing.
 

bruceb58

Solar Enthusiast
Ask Renogy. I know when they changed one of their battery designs, they did not recommend adding a newer design to the older ones. Likely the BMS had changed.

If the battery model number is identical, you will likely be able to do it but a call to Renogy should be made to confirm.

People were pretty pissed at Renogy with one of their batteries since if you got one replaced under warranty, the new model wouldn't play nice with the older model. Not a good thing if you already had a bunch of them in your existing system and only one failed.
 

robby

Solar Addict
If it's the same chemistry which it is and the voltage and Ah of all the packs match then it will work just fine.
You need to charge all the packs separately and then let them rest for an hour and check that the voltages are fairly even. If so you should not have a problem hooking up the three packs. The only pitfall is that you might age the newer battery slightly faster than if they had all been brand new.
 

Roswell Bob

Solar Addict
Thanks for your input Solar Addict. Each of these batteries has its own BMS for and internal breaker, so I'm not sure why I need to put fuses between them
In the rare instance that a cell fails a fuse is more likely to open than a breaker, relay or semiconductor switch. Suppose worse case you have one of the cells fail short. What you end up with is enormous amounts of energy (from all of the other cells in the array) trying to clear the faulted cell. A 280aH EVE cell has about 250micro ohms of internal resistance. The fault current would be near 12,000 amps with the power into the shorted cell well over 30kW. It is very unlikely that a breaker would open and I know for a fact that any semiconductor switch you use will fuse shorted before it clears(blows apart). The difficulty with turning off a fault lies with the parasitic inductance in the fault path. The inductive nature of the circuit causes large voltage transients across the switch as it opens. These transients will fail a semiconductor and cause arcing in a relay. I recently designed a semiconductor circuit breaker for a 600VDC/1000A for a client. The requirement was to clear 10,000 amps. I used almost 100 SiC mosfets in parallel along with some very large networks to keep the voltage under control when the switch opened. A BMS is not going to do anything to protect against a shorted cell. It will only add material to the fire. Your best bet is to protect each string with a good quality fuse.
 
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GXMnow

Photon Sorcerer
I just paralleled a new set of batteries with my original set that have about 80 full cycles through them. These are larger Li NMC cells, but the idea is similar. You do need to get them as close as possible to the same state of charge. As was suggested, using a separate charger to bring each one to full is a good idea. That should have them all vey close, but check the resting voltage before you connect the cables. Even 1/10th of a volt can mean a lot of current between the batteries. In my case, my new battery was charged higher than I normally run my system, so I ended up using a DC-DC buck converter to pull some power from the new battery and push it into the older battery which was still running my system. It took 12 hours to bring the new battery down to match the old one. I then paralleled them first through an 4 ohm resistor and measured the voltage across the resistor. This showed that I was well within 0.01 volt. So I then connected the cable direct and checked again with a DC clamp meter and saw less than 1 amp, so I was good to go. They have now been tied together and cycling for over a month.

The cables to the new battery are longer, and I used a cheaper BMS solution. I fully expected the old battery to carry a little more current. I was wrong. The newer cells with virtually no cycles apparently do have lower internal resistance. When I am charging at 30 amps, the old cells are taking 13.5 amps and the new pack is taking 16.5 amps. On discharge, I see the same thing as well. When the packs rest, they are hitting the same resting voltage and the current drops to true zero with no flow between the two packs. So the newer bank is moving about 10%-15% more watt hours than the older batteries to hit the same state of charge levels. So the new batteries are working harder, for now. Yes, it will age a little faster, but what should happen is the true internal cycle life will basically just "catch up" to the old bank and the current will end up balanced again at some point. This is not a big deal as I am pulling far less current than either bank could take on it's own. This would only become an issue if you wanted to pull 300 amps from 3 separate 100 amp rated batteries. That is a bad idea as they will never share he current perfectly. Adding another battery to get more run time is fine, but don't ask for more maximum current.

With my doubled up battery, I went from cycling 7.5 KWH a day to cycling 10 KWH per day. So both batteries are still cycling quite a bit less than what the one original battery was doing. I am only using less than 30% of my battery capacity on daily cycles. This should greatly extend the life of the cells. And I have a ton of reserve if I have a grid power failure.
 

Roswell Bob

Solar Addict
I just paralleled a new set of batteries with my original set that have about 80 full cycles through them. These are larger Li NMC cells, but the idea is similar. You do need to get them as close as possible to the same state of charge. As was suggested, using a separate charger to bring each one to full is a good idea. That should have them all vey close, but check the resting voltage before you connect the cables. Even 1/10th of a volt can mean a lot of current between the batteries. In my case, my new battery was charged higher than I normally run my system, so I ended up using a DC-DC buck converter to pull some power from the new battery and push it into the older battery which was still running my system. It took 12 hours to bring the new battery down to match the old one. I then paralleled them first through an 4 ohm resistor and measured the voltage across the resistor. This showed that I was well within 0.01 volt. So I then connected the cable direct and checked again with a DC clamp meter and saw less than 1 amp, so I was good to go. They have now been tied together and cycling for over a month.

The cables to the new battery are longer, and I used a cheaper BMS solution. I fully expected the old battery to carry a little more current. I was wrong. The newer cells with virtually no cycles apparently do have lower internal resistance. When I am charging at 30 amps, the old cells are taking 13.5 amps and the new pack is taking 16.5 amps. On discharge, I see the same thing as well. When the packs rest, they are hitting the same resting voltage and the current drops to true zero with no flow between the two packs. So the newer bank is moving about 10%-15% more watt hours than the older batteries to hit the same state of charge levels. So the new batteries are working harder, for now. Yes, it will age a little faster, but what should happen is the true internal cycle life will basically just "catch up" to the old bank and the current will end up balanced again at some point. This is not a big deal as I am pulling far less current than either bank could take on it's own. This would only become an issue if you wanted to pull 300 amps from 3 separate 100 amp rated batteries. That is a bad idea as they will never share he current perfectly. Adding another battery to get more run time is fine, but don't ask for more maximum current.

With my doubled up battery, I went from cycling 7.5 KWH a day to cycling 10 KWH per day. So both batteries are still cycling quite a bit less than what the one original battery was doing. I am only using less than 30% of my battery capacity on daily cycles. This should greatly extend the life of the cells. And I have a ton of reserve if I have a grid power failure.
Nice analysis.
 
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