running JBD BMS in parallel ?

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graham

Member
Joined
Jan 5, 2018
Messages
36
I have a 7s 30p that i put together with new cells, has been running well for over 2yrs with a JBD 60 Amp BMS it runs 24v dc pumps and controls for solar and wood heating.

I'm now looking to increase my capacity with a secondlife battery. To run a fridge in the summer, through an inverter, when i have excess solar power and possibly other 220v appliances. When the system was on Lead/Acid cells I had no problem with the load surge of the inverter it was just the DOD that i had to monitor.
Now I'm looking at the spec for the inveter and it states 120amp fuse on the DC supply to run the inverter at full potential. 1500w con 3000w surge.
The fridge that im looking to run is 0.49 Amp nominal an 85 kwh per annum so doesn't need anything near the full capacity of the inverter but starting surge is high.
So Im thinking of running 3x 60A JBD BMS in parallel
I have 700 cells at 2200mah - 2500mah
So 700/15 = 46P per pack to make 2 x 7S 45P to add in parallel to my existing 7S 30P PW.
I know i will have to be careful if one of the packs cuts out before the others, so was thinking 40Amp MCB between each BMS and the inverter.

Any thoughts or experiances?
Thank you
 

ajw22

Member
Joined
Nov 16, 2018
Messages
641
I believe this is your current setup, except for the upgraded 60A BMS?

I think you need to collect/provide more information.

So the fridge uses about 100W when the compressor is running. What about your 24V pumps and control units (fans?).
What's the Wattage of the other possible 220V appliances? 100W laptop or 1000W microwave?
Add all the Watts together to get your worst-case maximum consumption load.

I suspect that it's way below 1000W, thus just your current 60A BMS (which has a China-rating of 60A*24V = 1440W) should suffice.
So I think you can just tack on extra cells to your current 7s30p packs, if that's physically feasible. Also have to make sure your cables and connectors can handle the additional current.
If worst case consumption is above 1000W, you should probably start to consider a second BMS.
 

graham

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Joined
Jan 5, 2018
Messages
36
thank you for your input.

Yes sort of,, I have upgraded a few things, I now have a victron 15/75 and A 320w Mono panel,

The pumps and control gear runs at less then 1amp , Im looking to reduce that further by swapping out the old auto relays that i have controlling the pumps n valves.

The original spec for this was to run the pumps n valves for as long as possible in the depths of winter, It distributes the heat from our wood range. The scenario being we have had power cuts of over a week and if this happens in Dec/Jan in the bad weather we can have close to 0 peak sun hours , for what seems like forever,,

The original objective of the install is met, the added recharge capabilities of the 18650 cells with MPPT compared to the old lead/acid and the mono panel have greatly improved what power i can harvest in the winter.

So now i have an excess in the summer, I have a solar panel hotwater system which has a small control circuit and a dc pump this again uses less than 1 amp but in this case if there is sun for hotwater there is sun for elec so my current set up sits fully charged for most of the summer months.

I have had the inverter connected with only a 20 amp Auto fuse and been charging my elec bike and hand tools,
The fridge has a start up of well over 20 amps so pops the fuses, I have got some MCBs on the way to replace other auto fuses in my system, but wasn't sure what size MCB to use for the inverter or even if i could use an MCB not a fuse. From your suggestion of 1000w limit for the 60A BMS so a 40-45A MCB?

The other possible uses for my excess (lights, TV /PC and Freezer are taken care of by another solar installation) is kitchen tools, 500w mixer 850w toaster, 350w slow cooker, with the exception of the slow cooker all are used for short bursts.

From your suggestion of adding extra cells to my existing, practically it might be easier to build a new larger battery to replace my existing then strip down my original and include in a second battery.

Thanks again for your input,
 

ajw22

Member
Joined
Nov 16, 2018
Messages
641
The original spec for this was to run the pumps n valves for as long as possible in the depths of winter, It distributes the heat from our wood range.
Oh, we're going to need pictures of that setup! Seems like such a no-brainer to use a wood stove/range to heat water, and then to circulate that for various uses.

Auto relays use quite a bit of power (0.1A?), so modifying/changing them should reduce energy use.
A quick, good option might be to use a PWM motor driver module like this one. Here, you have the option of reducing the motor speed and save even more energy. If the motors need to run at 100% most of the time, a simpler DC SSR module might be better.

MCB: most are for AC only, and might not work well with DC. Check the specification or marking on the device. There will be markings such as "400V~" (wiggly wave) for AC, and something like "100V=" (2 parallel lines, one dotted) for DC.

The quick and simple solution to get just your fridge working is to just replace the 20A auto fuse with a 30A one. Your current parts do look like they could easily handle bursts of 30A, but just as an exercise:
* 30A split over 30 parallel cells => 1A peak/cell, OK
* 30A for a 60A BMS, OK
* 30A for 4~6mm^2 cables, OK
Now, to get your 500W mixer (peak 1000W~1500W?) and 850W toaster will require quite a bit more than 30A.


If you're going to rebuild and combine your battery, you might want to consider leaving space for future expansions. With my system, I left space for batteries and breakers, so that over time I could add up to 10x 14s100p batteries. Now after 2 years, got 7/10 batteries installed.
 

graham

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Joined
Jan 5, 2018
Messages
36
Thats great , Thank you for your advice, , I will give the 30 amp fuse a go to get the fridge going the other appliances will have to wait for now.
Good to have some real world advice from some one like yourself who has , (now that ive seen your build) used the JBD bms to great effect. What a great lookin project you have built, I will definitely be using it as a reference. Especially the software, its an area i have played with but not to great results.

I have a bunch of DC Tazin MCBs on the way but have read they are only reliable in the lower ratings.

One aspect that i need to address is the space and layout, you are right it would be best to make room for future expansion as i see you have done. I have another solar system that runs DC Freezer / lights / PC/TV. This has 24v system with 4x Pb Trojan T105 225ah C20 which I would like to replace in time with a secondlife powerwall, This is setup in an out building for safety and insurance. So plenty of room to expand. I see you are using 14s 100p. I'm thinking of going 7s 80p/100p I see from your setup that the JBD has no problems with this, what size bms are you using?

I will get some photos of my hotwater/heating system, Its simple but effective with just a 'Laing ECO-circ D5' well two really, one for the solar side and one for the hotwater/radiators, 28mm copper pipes allow for low speed settings on the pump. with the hotwater controlled by a two way motorized valve. After some teething problems its been very reliable.

thanks again
 

ajw22

Member
Joined
Nov 16, 2018
Messages
641
All my 7x BMS are 14s 60A, but I only ever use around 20A. I think I changed the parameters of the BMS to trigger at 28A.
Just in case, I have automotive 30A fuses. And for extra safety and convenient shutoff switch, 32A breakers.

24v vs 48v: for larger systems, 48v is definitely the way to go. You get 1/2 the Amps and 1/4 the resistive losses. Means cables, fuses, breakers, BMS can all be sized much smaller. If you have plans to significantly expand your system in the future, I'd make the switch to 48V sooner rather than later.

The BMS have performed as expected, except for 1 unit that has developed a weird balancing issue some 6 months ago. I've disabled the balancing feature on that one unit, and it's been working just fine that way. The cell voltages have slowly drifted apart, so I'll need to either do a manual balancing once or twice a year, or fix/replace the BMS.
 
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