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Questions for the confused ....

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Gould

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Jan 18, 2021
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I'm tired .... from reading this forum for 3 days straight!

Hey everyone, new to the forum and batteries in general. Typical story, I'm putting together a solar system for an RV and have slipped into the DIY battery rabbit hole. I'm trying not to be that guy who asks the same question you've heard a million times and have likely read it somewhere but honestly, my mind is in overdrive. I'm contemplating an order of cells but need a couple of things clarified before I pull the trigger.

1.) The core of the system will be a 3000W invertor. Unlikely I'll need that kind of power out of the gate but I want to build the system to support growth. The first thing I'm trying to is understand is the requirement of my batteries to reach this power output. Is the amperage limited by the cells or the BMS? If I build a single 4 cell pack can I add a 250amp BMS or am I constrained by the output of the 4 cell pack?

2.) Conversely regarding charging, what is the limiting factor to determine the amperage the pack is capable of handling, the parameters of the cells or the BMS?

Appreciate any assistance.
 
Welcome to the forum:

1) BMS more often limits the charge/discharge current vs. the cells. The second limit is the cells themselves. They have a continuous discharge rating. Most are 1C or 1X the capacity; 280Ah cell can't put out more than 280A continuously safely.

2) Same except charge limits are often 1/2 the discharge limits. Depends on cells and BMSs.

More importantly is have you established your energy needs, i.e., how much energy do you plan to use with the items on board? How much solar is available?

Link #5 can give you your solar hours for your location, panel orientation and tilt.
Link #1 is used to conduct an energy audit.

Once both have ben completed, designing the solar system/batteries is almost trivial.
 
Thank you, the links are much appreciated. I'll run through the content as quickly as I can. I thought I had plenty of time to pull this all together, until I stumbled on the option to build my own batteries. Apparently with the Chinese new year coming, orders are needed in days to avoid the lengthy shut down.

I'm sure I'll be back with more questions!
 
If you can conduct your energy audit and establish your daily kWh usage, that establishes your 24 hr battery need.
 
Gould said:
Thank you, the links are much appreciated. I'll run through the content as quickly as I can. I thought I had plenty of time to pull this all together, until I stumbled on the option to build my own batteries. Apparently with the Chinese new year coming, orders are needed in days to avoid the lengthy shut down.

I'm sure I'll be back with more questions!
Click to expand...

I think there are some forum members who buy quantities and ship them from within the US.
Look into that, might eliminate pressure to place an order quickly.
 
snoobler said:
If you can conduct your energy audit and establish your daily kWh usage, that establishes your 24 hr battery need.
Click to expand...
That will have to be my approach. I have a pretty good feel for our current usage but the impetus for this build is a trip to Alaska and down the Oregon coast. If in doubt I'll pad the numbers a little.
 
Hedges said:
I think there are some forum members who buy quantities and ship them from within the US.
Look into that, might eliminate pressure to place an order quickly.
Click to expand...
I'll look into that, thanks. The only complication is I'm coming to you from the Great White North - Alberta.
 
Gould - you have a couple of choices on bms types to fully power a 3000 watt inverter.

voltage: some people will create a 24 or 48v battery to power the inverter and a transformer down to 12v.

others will parallel three or four 12v batteries, each with a smaller bms.

others will get a bms that uses a contractor (big relay), and create one big battery (500 or 750ah) battery.
There are pros and cons on each approach- which do you favor?
 
This could me into trouble, I know just enough to be dangerous!

I've tossed around the idea of a 24v system to lessen the amperage I'd be dealing with and potentially lower the cost of some of the components. In the end I've settled on 12v for simplicity sake - this is all brand new territory so its all pretty complicated for me at the moment. From a cost perspective the savings really weren't there either to justify the complexity. The lower cost of the MPPT is offset by the higher cost of the 24v Multiplus. I'm sure there's some savings to be had in the smaller wiring but I haven't gone that far.

So herein lies the key decision, a large bank and bms or smaller paralleled banks? I was questioning if a single 4 cell 280ah pack could get me the 3000W. At a 1C discharge is sounds feasible but I'm not sure I'd want to be stressing them that way. I'm leaning towards 2 batteries each with a smaller bms - does a 150a bms exist (I've been leaning towards Overkill but they only go up to 120a). Going this route doubles my order so I'm trying to come to terms with that decision!
 
What inverter are you looking at?

does it have a surge rating?

what is the largest load you will run at one time?
Microwave + small stuff
A/C + small stuff
Power tools
 
Rocketman said:
What inverter are you looking at?

does it have a surge rating?

what is the largest load you will run at one time?
Microwave + small stuff
A/C + small stuff
Power tools
Click to expand...
I'm looking at the Victron Multiplus 3000. I like the idea of the invertor charger wrapped up into one, not to mention the assist it will provide if shore power can't keep up with the load.

In terms of loads, the largest will be the micowave for only minutes at a time, kettle, and coffee pot. I've considered running the fridge if we'll be off grid for a while, propane can be add up but not a necessity. The odd power tool, mainly a small compressor. I've contemplated the AC but that might be stretching things.

Hope to have a full list pulled together tonight. I've started and recognize there isn't much that I'd be required to run in parallel; no need to run the microwave while brewing coffee.
 
Gould said:
I'm looking at the Victron Multiplus 3000. I like the idea of the invertor charger wrapped up into one, not to mention the assist it will provide if shore power can't keep up with the load.

In terms of loads, the largest will be the micowave for only minutes at a time, kettle, and coffee pot. I've considered running the fridge if we'll be off grid for a while, propane can be add up but not a necessity. The odd power tool, mainly a small compressor. I've contemplated the AC but that might be stretching things.

Hope to have a full list pulled together tonight. I've started and recognize there isn't much that I'd be required to run in parallel; no need to run the microwave while brewing coffee.
Click to expand...

How small a compressor? Whatever the nameplate says, multiply by 5 to get starting surge.

Would be nice to have a priority switch, power strip with current transformer that detects current draw from microwave and temporarily switches off coffee pot. But a SPDT toggle switch feeding two outlets could take care of that.

Absorption fridge is a heavy draw when powered electrically, new models may be noticeable drain even with propane due to gas valve. Compressor types are much more efficient.

With enough PV, an A/C can be run. I do that at home. But an RV probably gets heated too much if parked in the sun, and PV panels don't work well in the shade.

If you don't have too little PV, you'll have too much. Would be nice to find a use for it.
 
And your audit should probably include the question: What can I convert to run off DC instead of paying the inverter tax? All kinds of electronics are easy since they already run on DC and thus you are paying the tax twice if you run them off an invertor. Microwaves and AC are probably off the list for that. Maybe you don't need either when you are in the RV.
 
That’s a great point, I think of things like the stereo and device chargers. Unfortunately (who am I kidding, the reason we got into RV’ing) the RV is a second home with many of the creature comforts. The microwave although not used often, is a must. AC is really only used when it’s absolutely necessary and usually only when we leave the dog for an hour or two. I’ll see what the numbers come in at to see if it’s worth it. Worst case I carry a Honda generator that could be used in conjunction with the Multiplus. A absolute last resort however. Even though this generator is the quietest out there, any noise is noise.

Getting closer to pulling the inventory together, I didn’t realize how much we depend on electricity. The unit is in storage so I won’t have the actuals but should give me idea of the hungry devices. Hoping/needing to make a call this weekend or our spring plans could be in jeopardy. Really appreciate the input all!
 
What kind of RV do you have? Trailer or MotorHome? Do you already have an old inverter ? How many batteries do you currently have?

I have the Multiplus 12/3000 in my MotorHome. Currently have 12v 440ah of lead batteries with 800 watts of solar on roof and mppt 100/50. Without starting generator, we can run microwave or hair dryer for short bit ( under 5 min) in the morning, as much as we want for lunch, and short bit at dinner. Plus electric blanket at night and small stuff 24/7. If under trees or bad solar day we run generator for an hour or so.

I am going to lithium this spring, I ordered 8 270ah lishen batteries. Going to connect then into a 2p4s battery (500ah useable). I chose a Batrium watchmon4 for the bms. This way I can fully power the inverter and also still use the emergency boost feature on the MotorHome to help start the chassis diesel motor. If I rewire a bit I figure I can run one A/C for a couple of hours.

Batrium uses a contractor(relay) to switch the power- so the full amperage of the batteries are available. Other bms’s (like overkill) internally switch the power so you have their ratings in amps. Overkill has a 120amp rating. If you create three 12v batteries (4s3p) running in parallel that will get 360amps- which will be enough to completely run the Multiplus plus some surge (but not use all the surge - which you probably would not need anyway). Two batteries w/ overkill bms’s will deliver 240 amps- which may or may not be adequate (but will not be using the full capacity of the Multiplus).

Now you also need to make sure you have space for everything. (this was one of my biggest limits for my battery)
 
Last edited:
Excellent context, thank you.

I have a travel trailer with 50 amp service. Picked it up in the fall so the system is your traditional OEM to get it off the lot, no inverter and a single deep cycle marine battery. If you're managing with 440ah of lead acid I feel anecdotally I'll be fine. My current plan is Multiplus 12/3000, 150/60, 800 watts of solar with room to move to 1200 watts and batteries ... well that's why we're here :).

I've been looking into the larger BMS's, the brand you mention pops up often. It's pricey but that isn't the reason for my hesitation, I am concerned about the risk and complexity of introducing an external relay. I haven't spent enough time researching them to understand how they handle failure, fail closed as an example. Curious, you're designing for a single bms to manage two batteries? Didn't know that was possible.
 
Actually it is a single big battery. 2P4S is connecting 2 cells in parallel, then those supercells are connected in series. The BMS watches each supercell - I have 4 blockmons for the four supercells. (FYI 4S2P is actually two 12v batteries connected in parallel - usually with 2 bms's).

The contractor I have is Tyco EV200AAANA. It is a 500 amp contractor. I am designing the system so it should never have to connect or disconnect large currents, but looking at the spec sheet it can disconnect a large load without damage, a massive load with damage, and an good sized charge without damage. I don't think it can fail closed (connected), for that to happen it would have to weld the contacts when connecting, I am installing a pre-charge connector (from Rec-Bms), so the capacitors on the Multiplus can't damage the contractor with the "Invertor spark". Also a 400amp Class T Fuse on the battery is actually the "weak" link in the system. (At $40 per fuse- hopefully it not being tested :) ). I have more faith in the external contractor than in the FET switches built into low cost BMS's.

The Batrium BMS can (will) connect with the CCGX and will tell the CCGX the size of load and charge current the battery can accept. The CCGX will pass that info the Multiplus and mppt's. So (for example), if the batteries are getting low (and the generator is not auto-starting), at lowest cell voltages of 2.9v, the Inverter will be told to stop discharging. The batteries should rebound to approx 3.0 to 3.1 volts. My 12v systems will still be running but at 2.8v the Victron Smart Battery Protects will cut 12v power to the rig. Then if the RV sits for a while (without getting any solar???) The BMS will go critical and turn off at 2.7v. Then hopefully I have weeks/months to get it fixed before battery damage occurs. (Since I don't leave my Rig for a long period of time with the invertor running this should never happen).
 
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