Battery Combiner for 2 mixed 48v banks?

Pharawer · May 2018

Hey guys I have an 48V off-grid system. Working Flawlessly for about 2 months now. Loving the community so far.

OK so I want to add a new battery bank of 8 new Trojan T105 (225AH 6V). My old bank were 8 Powefast GC2 (208AH 6V). The batteries are not exactly the same brand and capacity (AH) but Age should be similar and both are flooded. My first bank only has 2 months old. I have heard mixed stuff about combining banks of different manufacturers. Some people say that there wouldn't be any problem if the batteries are made of the same chemistry and age regardless if they have different capacity. Others say that because of the different internal resistance of the batteries they will charge or discharge and overcharge more than the other, hurting them at the long term.

So I wanted to do a battery combiner (selector) in order to charge them separately once every weekend; instead of just connecting them without any sort of control.

However there aren't any commercial battery combiners for 48V systems. The highest I have seen is 32V. So I decided to make my own battery combiner.

I made a diagram. I think it should work in theory. I would be using DC breakers as disconnects. I would use the breakers to turn on or off any of the banks at a given time and then change the charging settings at the CC.

Image: https://us.v-cdn.net/6024911/uploads/editor/5y/9093cvcgv6gx.png

For the Voltage Recharge Settings in Bulk, Absorption and Float I am currently experimenting with the voltages from this table below found from this website
http://www.cleanenergybrands.com/shoppingcart/knowledgemanager/questions/98/Interstate+Deep+Cycle+Batteries+Charging+Recommendation
Finding information about the Powerfast Brand proved to be very difficult. I made a few calls to interstate and they don't seem they have any detailed info about this battery. I tried calling the Sam's Club number from the battery label but it seems like it is a dead number.

Image: https://us.v-cdn.net/6024911/uploads/editor/ip/1553m4y3lpju.png

So I am still testing the settings from interstate batteries. Making a few tweaks here and there The way I have it right now is like this:

bulk 57v / absorb 59v (3 hours) / float 54v / equalize 61v. (1 hour)

So far It is working nicely. I try to monitor the gravity of each cell once every few days.
With these settings they have been in average 1.265 of gravity for each cell. Before, I had the default settings that came with the charger and the gravity was very low around 1.230 in some battery cells.

Since Trojan batteries have higher voltage requirements, once I combine both banks I plan to start from low voltage (the settings I have now) and check them daily while I raise the voltage settings so that the powerfast batteries dont get hurt and trojan don't get undercharged.

As far as i understand my wiring diagram shouldn't pose any issues with the charging and discharging of the batteries right? Batteries will not be charging and discharging at the same time the way everything is wired. The wire I am using is 4/0 AWG. The copper bus bar is 1/4" thick and the common bus bar is rated for 250 Amps

In the Charge Controller I would have 3 Battery Settings.
1. Bank 1 (208 AH)
2. Bank 1 + Bank 2 (208 + 208= 416 AH) I dont know if 433 AH it's safe as It would overcharge the smaller bank right?
3. Bank 2 (225 AH)

I would like to know if this setup is safe or what kind of problems would arise. I haven't found much information about this topic on the net yet so any input is appreciated. I hope this is not confusing. I will try to edit this post if it isn't clear enough. Thanks in advance.

Edit 1: I added the Diagram image. Last time I added it as an Imgur link but it didn't show up. Added some information about my voltage recharge settings and a table. I also edited some minor grammar mistakes and left out words and arranged a sentence or two so it it makes more sense.

mcgivor · May 2018

Combining batteries of different capacity in parallel theoretically wouldn't mean the smaller string would overcharge because the voltage is what determines current flow, being in parallel the voltage of both strings would be the same, the majority current would flow to the point of least resistance first, the larger capacity string, in essence self balancing.

In practice as opposed to theroy it's probably best not to parrallel differing capacity, and or differing age for several reasons, when all batteries are of identical age and capacity, it makes monitoring the ballance of charge/discharge simpler because both strings should be equal. As the older string begins to loose capacity, the current demand in absorption will be higher, possibly delaying the transition to float, at the expense of the new string, by subjecting it to a longer absorption than what is required.

The separate battery bank idea is probably the best if not a little cumbersome having to alternate between the two. The best solution would be to have separate charge controllers to maintain each bank separately, with a battery selector switch, still cumbersome, or two entire separate systems perhaps dedicating one for small overnight loads on a small inverter to conserve energy, just some ideas, since there is no mention of what your existing system is comprised of.

mike95490 · May 2018

It's tough enough to parallel batteries from the same lot. between 2 diff Mfg's I'd say good luck.

Estragon · May 2018

One possible concern is different initial specific gravity of electrolyte. Manufacturers may use different strength acid depending on plate alloy, balance of capacity vs longevity, etc. If different, the strings may need different charge voltages and/or absorb times. I don't think this is a safety issue though.

Using DC rated breakers to make/break connections should be okay. I wouldn't want to use a home-brew series of knife switches or whatever to make/break under load.

BB. · May 2018

More or less, it probably would work. If you don't have other options and the batteries are relatively inexpensive, and you monitor them (check specific gravity, voltage, etc.), they would function.

The difference in AH capacity is not what I would worry about (the capacity difference is not large here--Flooded Cell batteries can have upwards of 20% variation in capacity between "identical" cells). The bigger issue is the possible differences in chemistry--Different mfg. use different additives to the plates (to strengthen, reduce water usage, extend plate life, etc.) that can change their charging and discharging voltages a bit, and it is voltage levels that really determine the charging of the batteries (a 1/2 AH capacity battery will simply take 1/2 the current when charging or discharging, for matched batteries in parallel--Do not series connect different AH capacity batteries in series--That is not good)--Which could mean that one string may use a bit more water, or may be 5% less state of charge, less than ideal current sharing, or similar issues.

If the batteries are "cheap" (golf cart batteries are among the most cost effective), and you are not expecting more than 3-5 years of life--I would not "scrap" a good set of batteries. If this is your first time off grid--There is a possibility that you will kill your first set due to other issues (somebody leaves loads one when you go for a few days/weeks and discharges the bank to zero, friends/kids come by for a visit and kill the bank, you forget to monitor water levels and "boil" some batteries dry, etc.)...

-Bill

Pharawer · May 2018

Hey guys thanks for all the input. I edited my post so now it shows a lot more information of what I am exactly doing, It may have been a bit unclear. I am new to this forum so I was wondering how to add pictures, but I am glad i found it.

mcgivor said:

Combining batteries of different capacity in parallel theoretically wouldn't mean the smaller string would overcharge because the voltage is what determines current flow, being in parallel the voltage of both strings would be the same, the majority current would flow to the point of least resistance first, the larger capacity string, in essence self balancing.

In practice as opposed to theroy it's probably best not to parrallel differing capacity, and or differing age for several reasons, when all batteries are of identical age and capacity, it makes monitoring the ballance of charge/discharge simpler because both strings should be equal. As the older string begins to loose capacity, the current demand in absorption will be higher, possibly delaying the transition to float, at the expense of the new string, by subjecting it to a longer absorption than what is required.

The separate battery bank idea is probably the best if not a little cumbersome having to alternate between the two. The best solution would be to have separate charge controllers to maintain each bank separately, with a battery selector switch, still cumbersome, or two entire separate systems perhaps dedicating one for small overnight loads on a small inverter to conserve energy, just some ideas, since there is no mention of what your existing system is comprised of.

I understand that the best way to deal with two different battery banks is with a dedicated charger for each but it's to expensive for me at the moment. Maybe in the future I will do it that way. The way I am building my combiner is more like a selector (please see diagram photo). I would be able to choose to charge each bank separately if I am around, but since I work I would probably be able to do it only in the weekends.

Estragon said:

One possible concern is different initial specific gravity of electrolyte. Manufacturers may use different strength acid depending on plate alloy, balance of capacity vs longevity, etc. If different, the strings may need different charge voltages and/or absorb times. I don't think this is a safety issue though.

BB. said:
The difference in AH capacity is not what I would worry about (the capacity difference is not large here--Flooded Cell batteries can have upwards of 20% variation in capacity between "identical" cells). The bigger issue is the possible differences in chemistry--Different mfg. use different additives to the plates (to strengthen, reduce water usage, extend plate life, etc.) that can change their charging and discharging voltages a bit, and it is voltage levels that really determine the charging of the batteries (a 1/2 AH capacity battery will simply take 1/2 the current when charging or discharging, for matched batteries in parallel--Do not series connect different AH capacity batteries in series--That is not good)--Which could mean that one string may use a bit more water, or may be 5% less state of charge, less than ideal current sharing, or similar issues.

-Bill

Yes . The voltage settings are probably the biggest issue to deal with. As far as I understand Trojan's have an antimony composition on the plates or post. I don't know if that affects anything, Trojan's seem to have higher voltage recharge settings than your average battery. While finding any information about the Powerfast battery composition is almost impossible. My best bet would be to try and experiment, tweaking here and there until I find some happy medium lol. I edited my post regarding that subject.

I am feeling more confident now in trying this out. At least I know it may work more or less as long as I keep an eye on the batteries monitoring them and stuff like that. I eventually plan to get a dedicated charger or sell my powerfast bank to a friend so I can replace it with another Trojan bank so both banks are of the same brand, I would try to do that in the next month if possible so Age wouldn't affect the new bank. I live in Puerto Rico and after the hurricane the demand for batteries have sky Rocketed. Getting more Powerfast batteries proved to be impossible.

I would still like if someone can take a look at my wiring diagram to see if its safe (since I added the image today). I think it is safe but I may have been overlooking something. Thanks for the input in advance.

Estragon · May 2018

IMHO, the diagram shows a reasonably safe setup. The DC breakers may be polarised and should be installed properly so the + or line side is connected to the source of fault (not normal operating) current. The inverter and controller breaker sizes should be as recommended by device manufacturers. 250a strikes me as big for a 48v inverter, but...

If the controller, inverter, and one/both battery breakers were closed, you could charge and discharge at the same time, but I don't think it matters much for your purposes.

As long as you're checking SG and water weekly, it should work out ok. I'd be inclined to up the absorb time to 4 hrs from 3. I think undercharging is more of a risk than overcharging. Newish batteries shouldn't use a lot of water anyway, and as long as you monitor levels, the risk of overcharging is pretty minimal.

The 1hr eq is presumably an auto-eq setting. I'd turn that off, and eq manually as/when needed. When doing a manual eq, continue it until the earlier of when the weakest cell stops rising in SG in ~1/2 hr checks, or the sun goes down. If the latter, repeat the next day.

mcgivor · May 2018

Regarding the charge controller, does it have a remote temperature sensor, the ambient temperatures in Puerto Rico are probably higher than 25°C so voltage adjustments will be required if the batteries are closeri to 30°C.

Pharawer · May 2018

mcgivor said:

Regarding the charge controller, does it have a remote temperature sensor, the ambient temperatures in Puerto Rico are probably higher than 25°C so voltage adjustments will be required if the batteries are closeri to 30°C.

Yes it has a temperature sensor. And yes most of the time the temperature is around 30 degrees C, I haven't seen it raising higher than that yet.You really know your stuff haha. But 30C doesn't seem that high, the batteries are cool to the touch. My controller is a Conext Mppt 60 / 150 and the Inverter is a Conext Sw 4048. I am still wondering how will the CC adjust the temperature for the other bank if it only has one sensor.....The CC has a temperature sensor and the inverter has another. I will probably put one sensor in Bank 1 and the other sensor from the inverter in the other Bank 2; the inverter has a charger too if I have it on grid support (i have it as off grid though). Or It might be better to just put the Sensor from the CC in between the two banks. What do you think?

Estragon said:

IMHO, the diagram shows a reasonably safe setup. The DC breakers may be polarised and should be installed properly so the + or line side is connected to the source of fault (not normal operating) current. The inverter and controller breaker sizes should be as recommended by device manufacturers. 250a strikes me as big for a 48v inverter, but...

If the controller, inverter, and one/both battery breakers were closed, you could charge and discharge at the same time, but I don't think it matters much for your purposes.

As long as you're checking SG and water weekly, it should work out ok. I'd be inclined to up the absorb time to 4 hrs from 3. I think undercharging is more of a risk than overcharging. Newish batteries shouldn't use a lot of water anyway, and as long as you monitor levels, the risk of overcharging is pretty minimal.

The 1hr eq is presumably an auto-eq setting. I'd turn that off, and eq manually as/when needed. When doing a manual eq, continue it until the earlier of when the weakest cell stops rising in SG in ~1/2 hr checks, or the sun goes down. If the latter, repeat the next day.

Great to hear that buddy, I haven't seen any ++ sings in the breakers so I don't think they are polarized, the ones from the solar array were though.The breakers I am using for the diagram are Panel Mount (PNL), i think orientation doesn't matter with those. As far as I remember the 250A DC breaker was suggested on the manual from Schneider. I too think it's overkill but ohh well, I hope it's a one time purchase haha. My guess is that the breaker is probably meant to prevent damage on the inverter if a short occurs between batteries.

My concern with the charging and discharging at the same time is because I will only have one cable on the battery for the positive and the negative. Can the electricity pass in and out in a bidirectional way inside of the cable if I am charging and discharging the batteries at the same time? I understand that electricity passes from the highest voltage point (CC or/and Battery) to the lowest (Inverter or/and Battery). But I am still a little bit worried. Like if a cloud passes above my panels and the voltage of the CC plummets, the batteries will take the load right? in that case will the CC pass the little current it has to the inverter or to the batteries? if the answer is the batteries then how can it charge them if they are already giving energy to the inverter and there is only one cable. In my diagram only one cable is going to each bank for charging and discharging.

Thanks again for all the replies this has been super helpful. I will definitely try the 4 hour absorb setting and the eq tip. Cheers!

BB. · May 2018

Current flows "down hill"... If the batteries have the lower voltage, they are charging (charge controller it outputting more current than the loads are taking). If the batteries have a higher voltage, then they are discharging (loads are taking current from both the battery bank and the solar charge controller--if the sun is up). If the charge controller output current equals the load current in--Then zero current to flowing to/from the battery bank.

You cannot charge and discharge the battery bank at the same time.

In reality, the battery bank is what sets the "bus voltage" (and responds to sudden increases or decreases in current flow and/or direction). The solar charge controller is not designed to supply a fixed voltage (and variable current) directly to loads without the battery bank in the middle to "buffer" the instantaneous changes in current flow and direction.

-Bill

Pharawer · May 2018

Took me a few times reading that but I am glad I understood it haha. Thanks, the answer I was looking for is sorta implied there. The bus voltage explanation really helped me to make it click. So the batteries will be responding to external changes all the time, I was focusing in the wrong component haha. The learning never stops it haha.

I will be working on this during the week when I get all the materials. I will be updating the post soon

My respects and thanks to you guys.

Pharawer · May 2018

Update: Hey guys I finished setting everything up a week ago, its working great. Thanks again guys.

BB. · May 2018

Glad to hear things are running well.

If you have any questions, we are here.

Bill

AussieOffgrid · May 2019

I have a similar issue using two different AH AGM battery banks.... I to looked for a commercial combiner without any luck. Ended up with a Blue Sea battery switch on-off-on to change between banks. Any other suggestions would be greatly appreciated.

Battery Combiner for 2 mixed 48v banks?

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