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Old 02-12-2022, 10:23 AM   #1
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One big battery or bank of smaller batteries

Looking to go Lithium on our house battery, but here's the rub, they are EXPENSIVE, upfront.

I know the benefits long term, my main thing is spring for one big one, say a 200AH or start with a smaller 100AH and build from there.

We'll be a smaller rig, no inverter, running fridge, lights, pump, charging devices.

Is it better to have a multiple battery bank or one battery that suits your power needs?

Obviously slightly cheaper to buy one big one upfront, but I could spread cost and scale up if needed.

Thoughts?

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Old 02-12-2022, 10:55 AM   #2
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There are considerations when paralleling batteries in general, and additional considerations when paralleling lithium (regarding BMS's). There are also considerations with mixing old & new batteries in parallel arrangements.

One (not at all the only) way to navigate these considerations is start with a system that can switch between 2 batteries. That's what we're doing. We're starting out with one battery and a switch like this one that can switch between - but not combine - 2 batteries. All we have to do is make sure we have the space for another battery, and it allows us to easily upgrade in the future. This way, we can feel out the system, and find out if the additional storage will actually be needed before going all in.

Just have to make sure that each battery alone can sufficiently provide the continous current to power everything you see yourself running at once.
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Old 02-12-2022, 11:27 AM   #3
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I would steer the OP towards 400Ah AGM for their use case. TheHubbardBus' suggestion of an either/or switch is also solid advice. I think a central battery is easier to manage...
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Old 02-12-2022, 01:39 PM   #4
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Fault tolerant

Example 1.

I have an engine that can make hot engine coolant and that is used for the heaters in the bus. I also have a diesel fuel engine coolant heater. See? Two sources for heat. I can lose one heat source and still have heat.

Example 2.

I have two 12v refrigerator/freezers. If one quits working I won’t lose all my refrigeration.

Example 3.

I have two big, 8D batteries, for starting the bus. They can be used for house power if needed. I have two house batteries also.


Not very often will two separate systems fail at the same time. I recommend two house batteries instead of one. If one goes “bad” there is favorable probability that you can take the bad one out of the system and still function at a reduced capacity.

Two fresh water sources, etc.

I can charge batteries with the engine, with shore power, with a generator.

Get the idea. Every critical system has a back up.


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Old 02-12-2022, 06:21 PM   #5
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I agree strongly with the multiple failures required to shut you down approach. My battery bank is 6 150ah silicon AGM 12 volt batteries. Many battery salesman tried to steer me to 2v x 6 each as they said it was more efficient. I responded that I’d one battery failed I would be SOL. If one of my batteries fails I will have a smaller “gas tank” but still 12v to run the world. Busses have terrible suspension and vibration is bad for batteries.
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Old 02-13-2022, 04:37 AM   #6
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Redundancy is good but small batteries means more connections, each a potential problem. A couple of batteries is good. A bank of 10 is getting to be a lot of connections.
We're going with server rack lithium units. 12v/400ah or 24v/200ah or 48v/100ah. We're choosing two 24 volt units that will give 10.24 kWh for about 3300 shipped. They'll be configured in parallel for a 24VDC system. If one fails we can pull it out and send in in for warranty while still keeping 5.12 kWh of capacity online.
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Old 02-13-2022, 07:45 AM   #7
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Quote:
Originally Posted by kazetsukai View Post
I would steer the OP towards 400Ah AGM for their use case. TheHubbardBus' suggestion of an either/or switch is also solid advice. I think a central battery is easier to manage...
Can you explain why? I am willing to shell out the money for the lithium, but wanted to see about one big or several smaller(with approx capacity). Why AGM? They are so damn heavy. And can only support 50% DOD.
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Old 02-13-2022, 04:19 PM   #8
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Why would anyone go AGM unless they had very minimal needs or were looking at very short term retainment?
AGM has only 50% of the rated power available before damage occurs.
You can currently buy 400ah (at 12.8 vdc) of lithium for $1500 including BMS with a weight of 101 pounds and 7000 cycles to depth of dischage 80% while retaining (at the end of those 7000 cycles) 80% of the new spec.
That's 5.12kWh new and 4.1kWh after 7000 cycles to 80% depth....which by the way is NINETEEN YEARS


AGMS's even at a lowly (not top of the line for sure) price of $200/100ah would require 8 units (50% depth of discharge) cost $1600, require replacement at least four times in that 19 years, weigh close to five times as much, and take up 4 to 6 times the space.


Unless one has minimal or short term needs, lithium is the ONLY way to go unless you absolutely can't fit it into your budget.
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Old 02-20-2022, 09:53 PM   #9
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Quote:
Originally Posted by spenceadventures View Post
Looking to go Lithium on our house battery, but here's the rub, they are EXPENSIVE, upfront.

I know the benefits long term, my main thing is spring for one big one, say a 200AH or start with a smaller 100AH and build from there.

We'll be a smaller rig, no inverter, running fridge, lights, pump, charging devices.

Is it better to have a multiple battery bank or one battery that suits your power needs?

Obviously slightly cheaper to buy one big one upfront, but I could spread cost and scale up if needed.

Thoughts?

I now nothing about batteries, but I have a few thoughts that I would like to share.


MTBF - mean time between failures. At one time I was interested in redundancy & cost. Basically the more components you have in operation the smaller the MTBF becomes - you are going to have more failures because you have more components that can fail. If you have a failure in a critical system that fails and you die, then automatic, dupe systems operating at the same time is imperative and very expensive. If you have a failure that is recoverable by repairing/replacing a component and life goes on then it is much cheaper to do that. Components that have a proven longer/bigger MTBF can be more expensive initially but are often cheaper in the long run and can be a desirable compromise.


It seems to me that if you have 2 batteries and you are using one until it is exhausted (do you consider that a fail?) and then switching on the second battery, the first battery is going to get more wear than the second battery. You would almost have to alternate using one then the other all of the time to avoid prematurely killing the first battery (I'm thinking FLA here) and in that case having one big battery (Lithium?) would result in a bigger MTBF and lower cost and complexity. Complexity also breeds failures and increases cost. KISS.



In addition some components wear out even without use (think tires and FLA batteries). Perhaps having an alternate system like a generator would be a cheaper choice and provide more redundancy. As they say, only you can decide what is more important to you before you can make a decision.
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Old 02-21-2022, 08:12 AM   #10
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Quote:
Originally Posted by spenceadventures View Post
Can you explain why? I am willing to shell out the money for the lithium, but wanted to see about one big or several smaller(with approx capacity). Why AGM? They are so damn heavy. And can only support 50% DOD.
I'm not going to say you _shouldn't_ do Lithium, just that given your stated use case:

Quote:
We'll be a smaller rig, no inverter, running fridge, lights, pump, charging devices.
This doesn't really rise to the level in my mind where the tradeoffs that come with Lithium are all that worth it. Lithium is generally much less forgiving, and requires you keep an eye on it. All of the capacities you're looking for are easily doable with cheaper AGMs that can and will last a while with care, and as long as the system is setup correctly, will require little monitoring and little to no intervention.

You can also mount the AGMs somewhere outside of the rig if you so choose to save interior space... Aside from LTO, Lithium cannot tolerate charging in freezing temperatures. And discharging in freezing temps you lose a lot of efficiency.



Lithium has its advantages. I went with a very large lithium bank because I needed the central inverter, air con, etc and wanted more or less the experience you get from a small apartment. Its simply not possible to achieve the same energy storage capacity in a reasonably sized/weighted package using AGM. I also monitor the heck out of my main rig's power system. My battery compartment is insulated and heated.

OTOH I also have a short bus which has a 400Ah AGM bank + a single 300W panel for random tools, lights, etc. I never mess with or really look at it, it all just works when I go to use it. Its been set-and-forget.

Again I'm not going to knock you for going Lithium if you so choose, just my .02c.
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Old 09-15-2022, 09:29 AM   #11
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Redundancy is good but small batteries means more connections, each a potential problem. A couple of batteries is good. A bank of 10 is getting to be a lot of connections.
We're going with server rack lithium units. 12v/400ah or 24v/200ah or 48v/100ah. We're choosing two 24 volt units that will give 10.24 kWh for about 3300 shipped. They'll be configured in parallel for a 24VDC system. If one fails we can pull it out and send in in for warranty while still keeping 5.12 kWh of capacity online.
@hamskoolie - where did you decide to put the server rack?
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Old 09-15-2022, 01:41 PM   #12
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I have two entirely separate house battery banks, each one charged by its own charge controller that is powered by its own array of PV panels. In other words, I have two separate house systems. Each battery bank feeds through a 250A Schottky diode to electrically isolate it from the other, so if one battery or one bank went tits up it wouldn't suck power from the remaining good bank. The two banks are connected after their diodes to the main DC busbar that feeds all house loads. An incidental benefit of splitting my house batteries this way is that I can replace one bank at a time, and if one bank is producing less power than the other it doesn't matter much. I could even run different batteries in each bank, such as GC2s in one and L16s in the other.

Redundancy is good!

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Old 09-18-2022, 01:21 PM   #13
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@hamskoolie - where did you decide to put the server rack?

We really wanted the battery bank, inverter, etc. underneath the rig but settled on in inside wall mount. The battery rack will likely sit on a steel frame above the litter box bay with inverter and AC distribution box above the batteries. Most likely position will be midway down the drivers side but since we are now going with a 48 volt battery bank and inverter we can put it anywhere and not be concerned about voltage drop.
We are still in chassis prep mode (re designing chassis battery storage, deleting four more windows, removing a few dents, installing roof rack and solar panels.
The inside stuff can be worked on while it's cold and raining outside so the floor plan is still in flux
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