Is this the correct way to parallel batteries?

[jim furlong]

I have 2 48v 100AH 5kwh batteries (each with their own BMS) that I would like to put in parallel to have a 200AH battery. I've charged both batteries to within 0.2 volts of each other and let them sit for 24 hours. Now I think I can just connect the positive terminal on one battery to the positive terminal on the other and the negative on one battery to the negative on the other then take the positive from one battery and the negative on the other and hook it to my inverter.

I don't want to damage my batteries and get the most cycles out of it as I can.

I'm concerned about a higher SOC battery rapidly discharging into the lower SOC battery and the damage that might occur, if any

Thx

 

[snoobler]

Yes.

The closer they are, the better. I have a mental number of 0.1V. The surge is momentary as their voltages equalize very rapidly.

Once they are paralleled, make sure you use a resistor in series with the main leads to pre-charge the capacitors of the Inverter prior to making the final connection. That surge can blow the BMS(s).

 

[jim furlong]

Yes.

The closer they are, the better. I have a mental number of 0.1V. The surge is momentary as their voltages equalize very rapidly.

Once they are paralleled, make sure you use a resistor in series with the main leads to pre-charge the capacitors of the Inverter prior to making the final connection. That surge can blow the BMS(s).

I will get the batteries as close as I can before I connect

Thx for that tidbit about the resistor. I already have one but didn't intially and that spark let me know to respect a 48V system.

 

[Factory400]

200mV seems like a lot to make a hard connection.
For $9 you can get these resistors from Amazon

Comidox Set of 5 100W Watt Shell Power Aluminum Housed Case Wirewound Resistor 1R Ohm: Amazon.com: Industrial & Scientific

Comidox Set of 5 100W Watt Shell Power Aluminum Housed Case Wirewound Resistor 1R Ohm: Amazon.com: Industrial & Scientific

www.amazon.com

These will totally eliminate any guessing about a large rush of current while still make fairly short work of balancing them to the exact same voltage - where you remove the resistors and enjoy.

 

[jim furlong]

200mV seems like a lot to make a hard connection.
For $9 you can get these resistors from Amazon

Comidox Set of 5 100W Watt Shell Power Aluminum Housed Case Wirewound Resistor 1R Ohm: Amazon.com: Industrial & Scientific

Comidox Set of 5 100W Watt Shell Power Aluminum Housed Case Wirewound Resistor 1R Ohm: Amazon.com: Industrial & Scientific

www.amazon.com

These will totally eliminate any guessing about a large rush of current while still make fairly short work of balancing them to the exact same voltage - where you remove the resistors and enjoy.

Would my 25w 30ohm resistor work in that application? How long would I have to leave in connected between the 2 batteries ?

 

[Factory400]

That is too heavy to be practical. Even what I suggested is fairly over the top even if they are all in parallel.

You will probably find it easier to just get them a little closer - like the 100mV suggested at which point there is not much to worry about.

 

[snoobler]

Also, you can hook the inverter to the higher battery and pull it down.

 

[jim furlong]

I managed to get the voltages within 30mv and connect ed then without any issues or any current being transferred according to my multimeter. Thx for reassuring my original idea... Just wanted to confirm as I'm still learning. Next up is how to incorporate my 55 420w panels I got for a steal of a deal. I'll probably start a new thread on advice for that one.... Thx guys

 

[snoobler]

That's a lot of panel!

 

[Factory400]

55 panels? Crap dude......start your own utility, lol!

 

[Hedges]

10 kWh of battery.
20 kW of PV.
2C, 417 A charge rate.
What could possibly go wrong?

 

[jim furlong]

10 kWh of battery.
20 kW of PV.
2C, 417 A charge rate.
What could possibly go wrong?

I get that I have way too many panels for my measly 10 kwh pack... But I also have a 30kwh pack in my 2016 Nissan leaf and a 62 kwh pack in my soon to be 2019 leaf plus so with 100kwh's of storage I just need to fill in the gap between the packs and the panels to complete my off grid system. Any suggestions? I was originally looking heavily at the sol ark 12k but that's limited to 16kw of solar. I would like to use all my panels now that I have them. And 2 sol arks is way too expensive, even one is on the high end of my budget

 

[snoobler]

What @Hedges is saying, you better make sure you limit your battery charging current to not blow up your cells or blow out your BMS.

You have some technical challenges and hefty expenses ahead of you to fully utilize that much solar.

I don't have any suggestions at this time, though I suspect @Hedges will suggest you pepper your property with panels in all directions to smooth out the harvest throughout the day.

 

[SomebodyInGNV]

... Once they are paralleled, make sure you use a resistor in series with the main leads to pre-charge the capacitors of the Inverter prior to making the final connection. That surge can blow the BMS(s).

If that a concern for 12V batteries?

 

[jim furlong]

What @Hedges is saying, you better make sure you limit your battery charging current to not blow up your cells or blow out your BMS.

You have some technical challenges and hefty expenses ahead of you to fully utilize that much solar.

I don't have any suggestions at this time, though I suspect @Hedges will suggest you pepper your property with panels in all directions to smooth out the harvest throughout the day.

I'm gonna do a south array on my roof and an east/west ground mount. And maybe a dedicated car charging array/inverter/battery... I'm not entirely sure what is the most efficient and practical solution for my off grid place.... I'm still new to solar. The only thing I have working at the moment is my bluetti AC 200. It's nice to be able to just push an on button and plug in a few panels.

 

[Hedges]

I'm gonna do a south array on my roof and an east/west ground mount. And maybe a dedicated car charging array/inverter/battery... I'm not entirely sure what is the most efficient and practical solution for my off grid place.... I'm still new to solar. The only thing I have working at the moment is my bluetti AC 200. It's nice to be able to just push an on button and plug in a few panels.

Yes, multiple directions helps, could reduce peak to maybe 70% as much.

What you want is to capture all PV you can and use it for AC (or DC) loads such as running A/C, but regulate battery charge current.
Fast charging requires cooling and shortens battery life. All you need to do is recharge batteries by the end of the day, maybe 0.1C

Some systems like Victron I understand can use a battery shunt and adjust SCC output to keep battery current at target level. That way if inverter starts consuming power the SCC can crank up its current. I don't know how fast that is vs. BMS response time, e.g. if inverter is feeding a 5kW load that suddenly shuts off.

The SMA system I have also regulates battery current. PV is connected with GT inverters, AC coupled. Battery is connected to Sunny Island. I have mine set at about 80A, 0.2C for my 400 AH AGM battery. the AC coupled PV is several times that, and supplies AC loads plus battery charging. You can have twice as many watts of AC coupled PV (e.g. 24 kW of Sunny Boys) as battery inverter (12 kW of Sunny Island).

Recent eBay prices would be $5000 for two SI-6048 and $5000 for three Sunny Boy 7.7 kW -41 version.

I know less about using lithium with Sunny Island. Some people do that without data link, but a REC BMS is one it talks to.

 

[snoobler]

Victron ecosystem with a GX device and DVCC enabled can set a maximum charge current without limiting array production for loads.

Shocking to me anyway, the AiO units also have a similar feature built-in.

 

[jim furlong]

What @Hedges is saying, you better make sure you limit your battery charging current to not blow up your cells or blow out your BMS.

You have some technical challenges and hefty expenses ahead of you to fully utilize that much solar.

I don't have any suggestions at this time, though I suspect @Hedges will suggest you pepper your property with panels in all directions to smooth out the harvest throughout the day.

I'm gonna do a south array on my roof and an east/west ground mount. And maybe a dedicated car charging array/inverter/battery... I'm not entirely sure what is the most efficient and practical solution for my off grid place.... I'm still new to solar. The only thing I have working at the moment is my bluetti AC 200. It's nice to be able to just push an on button and plug in a few panels.

Victron ecosystem with a GX device and DVCC enabled can set a maximum charge current without limiting array production for loads.

Shocking to me anyway, the AiO units also have a similar feature built-in.

That sounds like the "smart load" feature on the sol ark, which bypasses the battery and goes straight to the loads.i like that idea as it doesn't put unnecessary stress on the battery.

My plan when there is excess Pv production to dump it into heating,cooling, and charging my car batteries. I'll have to check out victron as they seem to have that capability as well.
Is that a specific feature of certain systems or is that how solar systems work in general... Pv to load to batteries.... Or is it usually pv to batteries to load?

So many questions ?

 

[Hedges]

My plan when there is excess Pv production to dump it into heating,cooling, and charging my car batteries. I'll have to check out victron as they seem to have that capability as well.
Is that a specific feature of certain systems or is that how solar systems work in general... Pv to load to batteries.... Or is it usually pv to batteries to load?

Your "Dump Load" to soak up excess only works if it can always take the power. If dumped to a water heater, what happens when that reaches high temperature cutout? If dumping to any AC load, what happens if inverter can't continue to deliver, either because it derates on a hot day or is otherwise shut off?

If your dump load can't take the power, it flows to battery. BMS then has to disconnect.

Traditional off-grid systems were PV --> SCC --> Battery --> Inverter --> AC loads
Battery had to accept peak current coming from solar power. Holding optimum charge current for FLA wouldn't happen (except if excess PV, and SCC clipped at spec'd current)

Victron's ecosystem supports the above configuration while regulating charge current, by measuring battery current with a shunt and communicating.

SMA's AC coupling and some others are PV --> GT Inverter --> AC --> Inverter/Charger --> Battery
The inverter/charger is programmed for a maximum battery charge current. PV could be sized several times that, and only target battery current is delivered. GT inverter is commanded to adjust its output to match AC loads + charging.
That's what I have.

During AC load-dump (A/C or well pump turns off), it can take some seconds for Sunny Island to raise frequency requesting reduced power. I think SI momentarily sucks down excess power and shoves it into battery. If so, that could be a problem for BMS which has a strict current limit. Lead-Acid can take a momentary surge. SMA recommends 100 Ah @ 48V per kW of AC coupled PV inverter. I have several times that much PV (undersized battery). What I don't have is AC loads approaching inverter capacity, so haven't done such large load-dump.

 

[KBWaldron]

Now I think I can just connect the positive terminal on one battery to the positive terminal on the other and the negative on one battery to the negative on the other then take the positive from one battery and the negative on the other and hook it to my inverter.

Remember that the cables going to the inverter needs to carry twice as much current as the cables between the batteries. If your intent is to add more batteries in the future do not continue to connect in this manner. Instead use a star configuration with all positive battery terminals connected to a common positive bus bar, and same for the negative terminals. Then heavier cables from the busbars to the inverter and charge controller. Breaker (+) and ammeter(-) go in series with those heavier cables.