Using same battery for engine and house

[CompLexxx]

Ok thanks for all the great info guys!

If I have a backup generator I can always just charge the batteries if ever they drop too low to start. And with a monitoring system you can always make sure that doesn't happen. So having it separate still doesn't seem like a necessity.

But here is my new question. Could I plug 2x 100AH AGM batteries on the engine and relay either 1 or 2 large lithium 12v 200AH batteries inside the bus to those 2 batteries?

Will save me from having a double alternator, shore and inverter setup for the second battery set and just have a larger overall battery pack.

[NavyVet]

Originally Posted by CompLexxx

Hi, we just bought a bus that was previously converted to a party bus to continue into a full RV and I see that they plugged the internal batteries that charged a fridge, tv's, sound system and 110v outlets all on the same batteries of the engine. Is this recommended? Could I continue to build the RV in this way with at most upgrading to more powerful batteries or should I separate them and use a new set of batteries for the "house"?

Can only tell you what I did.
I use main batteries for starting my RV/Bus.
I use 2 100ah agm for the house.
Between the main and house batteries i use a relay.
When i start my RV/Bus the alternator charges the mains. Once charged the relay lets the alternator charge the house batteries.
I put an inverter to run everything inside off the house batteries.
When discharged to about 50% i stary the RV/Bus and recharge the house.
Been boondocked for 4 months now.
Works great

[CompLexxx]

Thanks that sounds like what I will do. Just wondering if it is okay to mix AGM with Lithium

[john61ct]

Originally Posted by CompLexxx

Since we are a family of 4 in it full time any more space is appreciated, there is also the simplicity aspect since all the wiring is already there and I won't have to go in and figure out which connections are which.



But if there is good reason to separate them I will.

The reason is to ensure not getting stranded out in whoop-whoop.

Lead goes out in the engine compartment.

LI House, but unless the living compartment is **always** heated in winter, still want an insulated box with silicon heating pads for pre-heating before charging at any decent current rate.

[john61ct]

Washer dryer is a bit much to expect off solar.

Carry an inverter genset like Honda eu2100i for stuff like that, and also supplementary charging.

Will pay for itself in a smaller bank, really essential in northern winters anyway, solar only puts out a tiny fraction then

[kazetsukai]

Originally Posted by john61ct

Washer dryer is a bit much to expect off solar.

This is for the large part correct. It can be done but the cost in terms of system size is quite high.

High efficiency appliances are a must, or propane. Especially for the dryer- either heat pump or condensation unit (and the condensation process takes a long time even though its draw is lower). Resistive electric dryer: it technically can be done, but it is such an extreme draw and will cost you and no other load will approach it. Sort of a waste.


Solar laundry I'd expect to do your loads mostly when there is full sun with efficient appliances, but even that is somewhat extreme. And in my opinion attempting to do it with 12V is insanity. 12V, everything will cost you more for no benefit, all of your AC loads will be far less efficient, more stressed and thus less effective at their jobs. Not to mention that the "12V" Lithium batteries you see out there on the internet with internal/hidden BMSes meant as drop in replacements for AGM are way more expensive per kWh than other options.

OP... Until we know more about the loads you want to support, specifically which loads you intend to power off solar and how much generator time is acceptable, etc, we cannot be sure but- I get the feeling you're way underestimating what you will need, battery-wise, for this whole thing. Two 12V 200Ah batteries will get you anywhere between two and to six hours of AC assuming you have a very efficient single unit split, decent insulation and ambient temperatures in the high 80's / low 90's. Many opt for two splits front/back this will consume more energy. Lots of variables.

I'm one of the few people around here seriously trying to run AC purely off of solar power. To put in perspective 12V 200Ah is 2400Wh, two of these would be 4800Wh. My current bank is 10600Wh, and it is not sufficient to support more than one night of air conditioning with my mini split. The new bank will be 43008Wh.

4800Wh will be great, even awesome for charging devices and running lights, laptops and the like. Maybe a TV and a game console, the microwave/blender every so often.... Not for running household appliances unless you're okay firing up the generator to run these.

[Rucker]

Originally Posted by CompLexxx

Ok thanks for all the great info guys!

If I have a backup generator I can always just charge the batteries if ever they drop too low to start. And with a monitoring system you can always make sure that doesn't happen. So having it separate still doesn't seem like a necessity.

But here is my new question. Could I plug 2x 100AH AGM batteries on the engine and relay either 1 or 2 large lithium 12v 200AH batteries inside the bus to those 2 batteries?

Will save me from having a double alternator, shore and inverter setup for the second battery set and just have a larger overall battery pack.

House batteries should be charged through a DC DC charge controller that limits how much juice is drawn from the alternator. Renogy makes a 20A and 40A version, and you may want to use something like that, designed specifically for Lithium batteries. No need for a double system-though you may want to check the capacity of the alternator to make sure you're not driving it too hard.

My system has a 200A alternator, charges the main battery at whatever it needs; then there's a voltage regulator that charges the second battery at 15A max. That's all chassis battery and wiring. I have a 20A DC DC charge controller for the Lithium house batteries.

I suppose I could put on the 40A device, but felt it would be smarter not to tax that chassis system too much. Just doubles my recharge time.

[NavyVet]

Originally Posted by CompLexxx

Thanks that sounds like what I will do. Just wondering if it is okay to mix AGM with Lithium

I wouldn't. AGM is different type batteries.
Should batteries the same

[CompLexxx]

Ok, the exact calculations were not made and I was just guessing, to be corrected when I finalize everything.

The real concern was is it possible to run all on the same battery system or not. 6-8x AGM batteries seems like the way to go. Will the battery system be okay with a parallel relay that is 2 to 3 feet away between the 2 outside batteries and the 4-6 inside? It will probably take quite thick wiring.

I am okay with running the generator for the laundry and AC. AC is only needed when we travel down to Mexico, but the sun is also a lot stronger there so I hope it helps compensate a bit. I don't mind filling the whole roof with solar eventually if needed.

[john61ct]

You do need to make proper calculations - nothing will ever be "exact" but pretty close ballparking

based on actually **measuring** each load device, from the battery bank side, in units of Ah per hour

then estimate your usage in hours out of each 24hr period

Coming up with an average Ah per 24hrs in worst case conditions. Call that Z.

Called an "energy budget" lots of templates out there.

Then, if trying to be "mostly solar"

with lead chemistry,

You need 6-8 times that Z Ah number in storage.

With a genset, FF energy on demand, then 3-4 x Z will be enough,

too low a bank capacity means running the genset too much.

Say you figure out you need 800Ah of storage.

Then 2500-3000W of panels might let you be "mostly solar" at least in summer or near the equator.

Under 1500W of panels you will be relying on your genset, paying lots for fuel.

And no matter how much solar panelage you have, it is unlikely your bank will be big enough for more than a few hours of solar. Again, expect to run the genset for that - and spend a lot on fuel every week.

Parking in the shade and staying at high altitudes

learning to live like the locals, getting your bodies acclimated

are more sensible strategies.

Originally Posted by CompLexxx

6-8x AGM batteries seems like the way to go

How many units you need depends on their voltage compared to your DC system voltage, and how much Ah capacity

and no judgement - needed in the initial design stage, not a "finalizing" detail - can be made without that detailed energy budget.

The battery chemistry type is a completely separate discussion, ideally determined **before** estimating capacity needed.

_______
Again, none of this has to do with your ability to start your engine - you need a backup for that. If it isn't a dedicated Starter battery, then what do you propose to give you redundancy?


> Will the battery system be okay with a parallel relay that is 2 to 3 feet away between the 2 outside batteries and the 4-6 inside? It will probably take quite thick wiring.

By using an IGN relay you're saying that the Starter will only be charged by the alternator?

You sure you won't want to use a DCDC?

An ACR/VSR automatic switch would at least be two-way, allow Starter to share the charge sources used for House when camping long-term not using the engine.

[CompLexxx]

Yes I know. These are not exact calculations. I can't make them yet as I will be receiving the vehicle next Monday and then I will start listing and shopping for what we want.

The engine is a 8.4L Cummins from a 2004 Thomas MVP. From my research it is a 200Amp alternator. At about what level would people consider upgrading the alternator for a more powerful one?

I'm not sure what you mean by IGN relay with the starter only being charged by the Alternator. Currently the engine batteries are plugged to the alternator, shore, inverter, lights, sound system. If you had bought this setup from someone (it was previously a party bus and we are transforming it to an RV) and wanted to add in a few household appliances, how would you do it for the simplest setup?

I'm ok with DCDC

I would have to learn more about what ACR/VSR is.

[john61ct]

IGN relays close when the engine is running. Manual switches are vulnerable to brain farts leaving you stranded.

It is not a matter of just adding load devices

that does not happen until you have

1. a suitable House bank

batteries designed for long life under deep cycling usage

completely different category from the little cheap ones designed for Starter usage.

2. Charge sources suitable for that bank, ideally a mix of AC-DC charger (for your genset and where grid power is available), multiple DC-DC chargers for your alternator (200A is plenty), and your solar, which hopefully will provide 80+% of your energy on average in sunny conditions, and

3. all the other infrastructure required, including ensuring the Starter and House circuits are connected to each other only when at least one charge source is active. That's what an ACR/VSR automates, or included in a good DCDC, soon as charging stops the two circuits get isolated from each other.

The selection of the load devices intended to run off stored power should be made for maximum efficiency,

and those that will run off your genset(s) like aircon, sized appropriately to do so, likely need slow-start devices.

The stereo and other "party" gear will get switched to run off House circuit.

Only the engine works off the Starter circuit.

Mission-critical Nav / Comms etc safety gear should be switchable to either side

Devices requiring heat ideally using propane.

Clothes drying done in the sunshine, maybe even washed by hand.

You cannot just take a first-world S&B lifestyle and squeeze it into an off-grid mobile rig.

If you aren't willing to make compromises, then expect to spend many tens of thousands rather than just one or two.

[CompLexxx]

"IGN relays close when the engine is running. Manual switches are vulnerable to brain farts leaving you stranded."

But wouldn't having a generator negate that problem if ever it happened?

We will be using cooking and debating if we use the fridge and hot water with propane. Also opting for not having a dryer and dry hanging. The rest will mostly be our laptops.

Thanks!

[Booyah45828]

Start batteries are designed for a brief high load, and then a rapid recharge. They won't tolerate a complete discharge, and they don't even like being discharged/charged repeatedly. If that's your intentions, you need to use either a deep cycle or agm battery.

Running a start battery completely dead once has a good chance of permanently ruining it.

I also prefer people use a dc-dc charger over a large relay, so as to not overtax the bus's charging system, but the relays are typically cheaper and will likely work problem free.

[kazetsukai]

Originally Posted by CompLexxx

We will be using cooking and debating if we use the fridge and hot water with propane.

Hot water, absolutely. That is the one thing I went propane with in our bus and no regrets. Fuel lasts a long time for that use, we've replaced a 25lb tank twice in 8 months.



Fridge, up to you. It helps to have a combo unit that can do multiple source (AC/DC/LP) but that can be expensive. The DC "chest style" fridges out there like the Dometic/Alpicool etc use a fraction of the power of a residential fridge.

Originally Posted by CompLexxx

Also opting for not having a dryer and dry hanging.

One user I saw had an efficient washer unit with the space above and next to it open to hang clothes- and they had a dehumidifier in that same space. They'd close off the space with a curtain and turn the dehumidifier on, removing the moisture in that little area making it a giant, albeit slow, dryer. Very cool idea I think.

[Booyah45828]

Any fridge that burns lp or natural gas is an absorption fridge, and those are horribly inefficient when ran off of electric, even more so off 12vdc. So if electric efficiency is important to you, skip the absorption fridge and go with a compressor model.

[CompLexxx]

As I wrote I will be switching to all AGM batteries - or all Lithium if I relay them to the interior of the bus.

As for fridge I was looking into having a normal 110v chest freezer and putting a temperature controlled power controller on it that turns power off when it reaches fridge temperature, someone said on here that since they are super well insolated that those chest freezers can have about the same energy requirements as a 12v fridge, but cheaper entry cost and more space. I have all the setup for it so I will try it out and see how it fares.

Kaz was that dehumidifier a 12v or 110v? Basically making a little box around a clothes hanger could work. I was just planning on having it outdoors in the summer and indoors in winter is already dry so the moisture in the air only helps.

[Sehnsucht]

Originally Posted by CompLexxx

Kaz was that dehumidifier a 12v or 110v? Basically making a little box around a clothes hanger could work. I was just planning on having it outdoors in the summer and indoors in winter is already dry so the moisture in the air only helps.

I was thinking the same thing. Another idea may be to have the furnace duct routed through this 'airing cubbard', probably on the output side so that furnace heat is moisturized by the wet laundry then distributed into the living space. I don't have the luxury of a dedicated cabinet for this function but in my semi's sleeper I hang a wet towel at night that is dry by morning and it really helps combat the cripplingly dry air that results otherwise. Just in a semi bunk it can evaporate 20-30 ounces of water a night.

[john61ct]

The portable 12/24V compressor fridges are expensive, but very energy efficient, 200W solar should cover the consumption of at least one if not two units most of the time.

A depleted lead battery takes at least seven hours to be fully recharged, regardless of the amps output capacity from its charge source.

As stated, the Starter battery should not be allowed to deplete at all, just dedicated to cranking, maintained at 100% all the time.

If a second "backup / reserve" House bank is used for that function, then a good SoC meter paid close attention by a knowledgeable human will be needed to ensure not getting stranded, but IMO all that is overkill, only for remote "expedition" scenarios where survival is an issue.

[kazetsukai]

Originally Posted by CompLexxx

Kaz was that dehumidifier a 12v or 110v? Basically making a little box around a clothes hanger could work. I was just planning on having it outdoors in the summer and indoors in winter is already dry so the moisture in the air only helps.

I believe they went with a 110V dehumidifier. They do use some power depending on size, but its manageable.



As for humidity, you'll likely be doing your best to keep humidity low year round due to condensation. A dehumidifier of sufficient size is a plus in this regard too, you can dehumidify the whole rig with something the size they used IIRC.