Battery capacity

[Freebirds]

We do quite a bit of boondocking.  I'm not well versed in the electrical side of things, though.  Right now we have 2 6-volt deep cycle batteries that will get us through the night, but just barely.  I would like more storage than I currently have.  I don't want to add 2 more 6 volt batteries due to the weight.  If I get 2 200ah batteries, how much longer will that last vs the setup I have?  I know I have to change the charger if I go this route.  Is there another option?  Anything else I should be thinking about?

 

Thanks.

[Randyretired]

The two six volt batteries depending on size are around 200ah.  With lead acid it is recommended to only use half of the capacity for longevity.   This means you truly only have 100ah.  2 LifeP04 batteries with 200ah will  provide 400ah total if you use all of it.  Using all of the capacity isn't usually  recommended.  For one you don't want to wake up to a dead battery.  The BMS will cutoff and some are not easy to restart.  So I plan on 80%.  This would be 320ah or roughly 3 times the battery power you now have.  Before I would purchase a new charger I would measure the voltages the old charger  produces.  My old converter will charge to 14v.   That is nearly a full charge and is a full charge given some time.  Different manufacturers have different recommendations so you might see what the new batteries require.  LifeP04 do not need to be fully charged often like lead acid.  In fact the batteries will last slightly longer if not fully charged.  Long term if not fully charged there might be some balance issues but it all depends on how the BMS is setup.  Again check with the manufacturer to see what is needed.

[Kirk W]

3 hours ago, Freebirds said:

  I'm not well versed in the electrical side of things, though. 

I suggest that you take some time to read "The 12V Side of Life" parts 1 and 2. This was written by Mark Nemeth, the technical advisor of the Escapees RV Club. I have known Mark for a long time and he is a very knowledgeable and experienced RVer and a talented writer.

[Randyretired]

14 hours ago, Kirk W said:

I suggest that you take some time to read "The 12V Side of Life" parts 1 and 2. This was written by Mark Nemeth, the technical advisor of the Escapees RV Club. I have known Mark for a long time and he is a very knowledgeable and experienced RVer and a talented writer.

This was an excellent write up in its day but needs to be updated.  

[oldjohnt]

On 7/26/2023 at 3:54 PM, Freebirds said:

If I get 2 200ah batteries, how much longer will that last vs the setup I have?  I know I have to change the charger if I go this route.  Is there another option?  Anything else I should be thinking about?

Good questions, I agree with Randy's excellent advice.

  Those two 6 volt are maybe 200 to 225 Amp Hours and being lead acid gives you maybe 50% or 100 USEABLE Amp Hours.. HOWEVER, Two 200 AH of Lithium which is 400 AH and 80% of that (what I shoot for like Randy) yields 320 USEABLE AH.  In addition to their light weight and faster charging, I like how Lithium gives less voltage sag at decreased SOC versus lead acid. 

 I have no idea if your current charger can be set/adjusted for best charging Lithium ??? If not I recommend a new Lithium suitable unit.

Anything else I should be thinking about ?

 Charging ?? Of course Solar in addition to any new charger which requires a Solar Charge Controller and if needed consider a DC to DC charger so the engines alternator can help charging when driving, especially when there's no sunshine.

 BATTERIES; There are a bunch of players out there, more expensive as well as some cheaper alternatives, beware of Class B cell use, check out DIY Solar with Will Prowse on You Tube.  Consider some of the SINGLE UNIT higher AH batteries (300 to 400 AH) to reduce all those extra connections and I like a BMS that offers low temp protection, some batteries even have internal heating if low temps are an issue.

INVERTER: If you need a 12 VDC to 120 VAC Inverter and a new charger also, consider a COMBINATION INVERTER CHARGER to simplify wiring

 John T

Edited July 27, 2023 by oldjohnt

[Freebirds]

Thanks for the replies.  I will definitely be checking out the 12 Volt Side of Life.  The combination inverter/charger seems like a good idea.  I have a suitcase  160 watt solar panel with the controller on the back.  I have seen DC to DC chargers with various amp ratings.  How many amps should I be looking for?  Thanks again for all the advice.

[JimK]

I also agree that the 12v side of life is a good beginner's resource.  Just two comments:  first to save battery weight, going to lithium batteries is the only choice.  There are a lot of negatives including cost.  Second, the best single step you can take is to start with an energy audit.  Many people start out trying to live as they do at home.  Instead some simple changes can make a huge difference.  I boil water on my stove and use a mellita coffee system instead of needing an inverter and electric coffee pot.  I don't need or even have an inverter.  

[oldjohnt]

On 7/28/2023 at 2:40 PM, Freebirds said:

How many amps should I be looking for?  Thanks again for all the advice.

The answer DEPENDS. First on your house battery capacity and charging requirements (how many charging amps for how long ?? Is it the ONLY means of charging house batteries with engine running ?? Any rooftop solar when driving to help ??)  Second on the size of your alternator and how much you are willing to draw from it without risking over working it. Typical sizes may be 20, 40 or 60 Amps and its absolutely necessary your wiring, connectors and overcurrent protection be adequate and proper. Consult the manual. PS I don't like a control scheme that activates the DC to DC the instant the engine starts, I prefer to give the engine battery some time to recharge and stabilize BEFORE drawing say 30, 50, to 70 or more Amps from the engines alternator to charge the house batteries.

On 7/28/2023 at 2:40 PM, Freebirds said:

The combination inverter/charger seems like a good idea

  If you need BOTH and aren't already partly equipped  ?? it makes wiring easy. Lots of choices bells n whistles out there. A pass through hybrid sounds inviting with all sorts of choices limits and control schemes, it's fun spending your money lol

John T

Edited July 30, 2023 by oldjohnt

[kb0zke]

"INVERTER: If you need a 12 VDC to 120 VAC Inverter and a new charger also, consider a COMBINATION INVERTER CHARGER to simplify wiring"

I'm a contrarian here. I prefer single-job devices. I'd rather have a separate converter and a separate charger. That way if one of them goes down the other is still there. The downside, as mentioned, is that a combination device saves weight and space.

[oldjohnt]

17 hours ago, kb0zke said:

I'm a contrarian here. I prefer single-job devices. I'd rather have a separate converter and a separate charger. That way if one of them goes down the other is still there.

  You get NO argument from me I HAVE SEPARATE UNITS MYSELF. However they were installed some time back one at a time as I was building my system and  already had a quality Lithium rated charger BEFORE the combo units were as much in the spotlight. If I were starting all over Id consider a combo unit (space, weight and ease of wiring) but agree if it goes down I risk loosing BOTH charging and Inverting grrrrrrrrrrrrrrr I could rely on my DC to DC and solar charging which typically suffices but the lack of an inverter could be a pain. Of course, to each what works best for THEIR RV, it's their money their choice but we can have fun spending their money lol

 Nice sparky chatting with you, hey I'm often contrary myself, ask my wife  lol

 John T

 

Edited July 31, 2023 by oldjohnt

[Freebirds]

On 7/29/2023 at 9:17 AM, oldjohnt said:

The answer DEPENDS. First on your house battery capacity and charging requirements (how many charging amps for how long ?? Is it the ONLY means of charging house batteries with engine running ?? Any rooftop solar when driving to help ??)  Second on the size of your alternator and how much you are willing to draw from it without risking over working it. Typical sizes may be 20, 40 or 60 Amps and its absolutely necessary your wiring, connectors and overcurrent protection be adequate and proper. Consult the manual. PS I don't like a control scheme that activates the DC to DC the instant the engine starts, I prefer to give the engine battery some time to recharge and stabilize BEFORE drawing say 30, 50, to 70 or more Amps from the engines alternator to charge the house batteries.

  If you need BOTH and aren't already partly equipped  ?? it makes wiring easy. Lots of choices bells n whistles out there. A pass through hybrid sounds inviting with all sorts of choices limits and control schemes, it's fun spending your money lol

John T

I'm glad to help you spend money vicariously through me.   I'm planning on installing 2 200 amp/hr. lithium batteries.  My truck is the only means of charging the batteries with the engine running.  No rooftop solar - yet.  I have a 600 watt inverter already installed but want to upgrade that.  The more I think about it, keeping the units separate makes more sense for us.  Not sure about the limits of my truck alternator.  Where can I find that information?

Keep the ideas coming, keeping in mind, funds are limited.

[oldjohnt]

6 minutes ago, Freebirds said:

Not sure about the limits of my truck alternator.  Where can I find that information?

  Maybe there's a Serial, Make or Model number hidden somewhere on the alternator????  Maybe some of the trucks specs if you have any ?? Maybe communicate the VIN with the manufacturer or even somewhere on the internet ??

 If it's a diesel ?? its probably a hefty rated alternator, at least 100 Amps probably  more ?? 

 A PURE GUESS not knowing your truck (gas or diesel?) or alternator I would think at least a 40 Amp to a 60 Amp DC to DC charger is THE BARE MINIMUM that may suit you, perhaps even larger if available and you have a diesel with a high capacity alternator. Ive seen some of the less expensive (Renogy) DC to DC that activate via the key switch or ignition etc soon as ON while other more expensive (maybe Victron) are programmable, and of course they need to have switches/settings/programs for Lithium. I have a 40 Amp Renogy manually activated feeding 412 AH of lithium for driving (Ford V10 Gas) on cloudy rainy days, but typically my rooftop solar satisfies all my needs. Think Amp Hours of energy you require and how many Amp Hours a DC to DC might supply  ?? Say you drove 2 hours with a 60 Amp DC to DC charger, thats 120 Amp Hours IN THEORY subject to battery SOC and other factors.

 Not knowing your specs or needs Id think at least a 2000 Watt PSW Inverter may be a good choice, but if you needed to run some sort of an AC, then 3000

 Sorry I can't GUESS more absent your specs and your energy requirements, so take this with a grain of salt NO WARRANTY  See what the more experienced gents have to say

 John T 

 

[rickeieio]

And don't forget, the wire running to your plug is likely a limiting factor too.  Trying to maximize the power out of your alternator may shorten it's life considerably.

We're up against a similar problem, and it's worse if the truck is parked far away from the camper.  It doesn't happen often, but it does happen.  A small genny may be in your future.

 

Edited August 2, 2023 by rickeieio

[D&J]

What we do because we travel with a residential frig and a small chest freezer is run a 2000 watt inverter and keep the batteries changed with the truck when driving. When dry camping for the night I plug our converter into a old 650 watt Honda generator so we have tv and satellite and charge the batteries. The little Honda works hard but it was free and still runs great and after the batteries topped off it just idles along. When we go to bed the chest freezer gets turned off and the frig runs all night on the batteries without any problems. If we were going to boondock we would eliminate the freezer and get a small solar panel. We are running two AGM Optima 75ah Blue top batteries and all the lights we use have been converted to LED. We also have a 5KW built-in generator that is very rarely used, when it is used it's for short periods.

Denny

[Freebirds]

20 hours ago, oldjohnt said:

  Maybe there's a Serial, Make or Model number hidden somewhere on the alternator????  Maybe some of the trucks specs if you have any ?? Maybe communicate the VIN with the manufacturer or even somewhere on the internet ??

 If it's a diesel ?? its probably a hefty rated alternator, at least 100 Amps probably  more ?? 

 A PURE GUESS not knowing your truck (gas or diesel?) or alternator I would think at least a 40 Amp to a 60 Amp DC to DC charger is THE BARE MINIMUM that may suit you, perhaps even larger if available and you have a diesel with a high capacity alternator. Ive seen some of the less expensive (Renogy) DC to DC that activate via the key switch or ignition etc soon as ON while other more expensive (maybe Victron) are programmable, and of course they need to have switches/settings/programs for Lithium. I have a 40 Amp Renogy manually activated feeding 412 AH of lithium for driving (Ford V10 Gas) on cloudy rainy days, but typically my rooftop solar satisfies all my needs. Think Amp Hours of energy you require and how many Amp Hours a DC to DC might supply  ?? Say you drove 2 hours with a 60 Amp DC to DC charger, thats 120 Amp Hours IN THEORY subject to battery SOC and other factors.

 

 Sorry I can't GUESS more absent your specs and your energy requirements, so take this with a grain of salt NO WARRANTY  See what the more experienced gents have to say

 John T 

 

I have a Chevy 6.0 L gas engine.  The alternator looks pretty small.  I tried to attach a picture but I couldn't get it small enough with out compromising the quality too much.  We don't have a lot of energy requirements.  2 6 volt deep cycle batteries meet our needs, but just barely.  

If the engine charges the 6 volt batteries I have while driving, wouldn't it do the same for the lithium batteries without a DC to  DC charger?

[D&J]

28 minutes ago, Freebirds said:

I have a Chevy 6.0 L gas engine.  The alternator looks pretty small.  I tried to attach a picture but I couldn't get it small enough with out compromising the quality too much.  We don't have a lot of energy requirements.  2 6 volt deep cycle batteries meet our needs, but just barely.  

If the engine charges the 6 volt batteries I have while driving, wouldn't it do the same for the lithium batteries without a DC to  DC charger?

Yes it will but not to full charge because of the voltage difference between the to kinds of batteries.

Denny

[oldjohnt]

3 hours ago, Freebirds said:

If the engine charges the 6 volt batteries I have while driving, wouldn't it do the same for the lithium batteries without a DC to  DC charger?

NOOOOOOOOOOOOOOO NOOOOOOOOOOOOOO don't try that you might fry the alternator  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! NOOOOOOOOOOOOOO

 Unlike lead acid, a Lithium Battery exposed to unregulated unlimited charging  can draw huge current that may damage the alternator ..........

 A smart regulated DC to DC charger suitable for Lithium will limit current draw from the alternator yet still properly charge your lithiums and NOT fry the alternator. My alternator has worked fine that way to charge my lithiums when driving...

 If BOTH your engine and RV batteries were lead acid it can work but again DO NOT hook your alternator direct to Lithiums

 Based on what you say a 20 Amp or 40 Amp DC to DC charger if set for Lithium charging might get you by to charge lithium house batteries AND BE SURE TO USE ADEQUATE SIZED PROPERLY OVERCURRENT PROTECTED WIRE AND CONNECTORS from alternator to DC DC and to battery

 John T

 

 

[D&J]

1 hour ago, oldjohnt said:

NOOOOOOOOOOOOOOO NOOOOOOOOOOOOOO don't try that you might fry the alternator  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! NOOOOOOOOOOOOOO

 Unlike lead acid, a Lithium Battery exposed to unregulated unlimited charging  can draw huge current that may damage the alternator ..........

 A smart regulated DC to DC charger suitable for Lithium will limit current draw from the alternator yet still properly charge your lithiums and NOT fry the alternator. My alternator has worked fine that way to charge my lithiums when driving...

 If BOTH your engine and RV batteries were lead acid it can work but again DO NOT hook your alternator direct to Lithiums

 Based on what you say a 20 Amp or 40 Amp DC to DC charger if set for Lithium charging might get you by to charge lithium house batteries AND BE SURE TO USE ADEQUATE SIZED PROPERLY OVERCURRENT PROTECTED WIRE AND CONNECTORS from alternator to DC DC and to battery

 John T

 

 

I don't know about GM but Fords 12V circuit is fused and with the voltage drop on our Ford the best I can get back to the trailer is 12.8 so that would regulate the current draw charging the Lithium batteries. I can't get a full charge in my AGM batteries because of voltage drop so I'm sure the Lithium would be the same, basically it just holds the voltage after it drops to 12.6 to 12.8. 

Denny

[oldjohnt]

Freebirds, here's some more info for you to consider:

Some (like mine) brands of 12.8 Volt Lithiums call for charging at 14.4 to 14.6 Volts, check to see what yours are ?????? If you only have 12.8 Volts back due to line voltage drop they may not achieve a good full quality charge.  Thats a good thing about using a smart regulated Lithium suited DC to DC charger like mine. While NOT over taxing the alternator it electronically uses LIMITED raw alternator energy to charge the lithiums at their required 14.4/14.6 Volts.

Regardless of line voltage drop (say only end up with 12.8 volts) or any built in alternator current limiting I DO NOT recommend wiring an alternators output direct to Lithium batteries, but instead use a DC to DC Lithium suitable charger that will provide a smart regulated charge at the recommended 14.4 to 14.6 volts OR WHATEVER YOUR BATTERY MANUFACTURER RECOMMENDS ???

 But hey sure even at less than recommended voltage your alternator might still provide at least SOME DEGREE of charge to your lithiums, even if not perfect or recommended SO LONG AS THE ALTERNATOR DOESNT FRY because the lithiums drew too much current and the alternator lacked the necessary limiting to prevent such. 

 To be safe I recommend you check the lithiums recommended charging parameters and the limitations of your alternator (or if its okay to connect direct to lithium ?) 

 Best wishes CHECK WITH YOUR LITHIUM BATTERY MANUFACTURER and Alternator specs BEFORE deciding to use a DC to DC or wire direct. Hey a DC to DC isnt all that expensive but its YOUR money YOUR risk.. PS while I recommend use of a lithium suited DC to DC charger for the engines alternator to charge house lithium batteries, do whatever YOU like.......

 John T

 

Edited August 3, 2023 by oldjohnt

[Randyretired]

12.8v is not high enough to provide much charging into a lithium battery.  By the time lithium batteries are down to 12.8v there is less than 20% SOC.  Given enough time 12.8v will charge a lithium battery to around 17%.  It may take days to reach that charge. When my lithium batteries drop to a resting 13v I am looking for the generator.

Edited August 4, 2023 by Randyretired

[D&J]

9 hours ago, Randyretired said:

12.8v is not high enough to provide much charging into a lithium battery.  By the time lithium batteries are down to 12.8v there is less than 20% SOC.  Given enough time 12.8v will charge a lithium battery to around 17%.  It may take days to reach that charge. When my lithium batteries drop to a resting 13v I am looking for the generator.

Lithium may charge different off the truck than our AGM batteries and the voltage may be higher. Mine are always under a load from my refrigerator, chest freezer and disk brake actuator, if they are all off at the same time it may be higher adding charge to the batteries I'm not back there watching it, I just look at it after we stop. Flooded cells, AGM and lithium all have different charging and discharging characteristics and floating voltages. I haven't used a flooded cell battery in a trailer for 20 years so I have no idea how they would act but our AGMs stay up but not to full charge because of the load we put in them, if nothing was running I'm guessing they would come close to full charge but that's not going to happen, that's what the generator is for. We are only on our second set of Optima AGM batteries in 20 years so I'm happy with them, this set is 7 years old 

Denny

Edited August 4, 2023 by D&J

[oldjohnt]

13 hours ago, Randyretired said:

12.8v is not high enough to provide much charging into a lithium battery. 

I sure agree Randy, in order to pump any significant charging amps INTO a battery the source voltage NEEDS TO BE HIGHER THAN BATTERY VOLTAGE and for a 12.8 Volt Lithium, that's MORE than 12.8 Volts....My DC to DC charger (via its internal electronic circuitry) delivers 14.4 volts (what my battery calls for charging) to my lithiums while NOT over taxing my engines alternator, again I do NOT recommend wiring lithiums direct to an alternator designed for charging lead acid BUT THATS JUST ME do whatever you like. Its been fun over 50 years of RVing to see technology improve as I went from Lead Acid,,,,,,,,to Deep Cycle Lead Acid,,,,,,,,to AGM (still lead acid technology, difference in electrolyte) then Lithium yayyyyyyyy Im NOT going back either lol

4 hours ago, D&J said:

Flooded cells, AGM and lithium all have different charging and discharging characteristics and floating voltages

AMEN, Dito and X2 to that D & J !!!!!!!!!!!   I used AGM several years back, they performed great  !!!

  John T

 

Edited August 4, 2023 by oldjohnt

[Ray,IN]

There is a difference between obtaining maximum life from LA batteries and the percentage of draw-down one can use. The maximum life from LA batteries is obtained if you never draw them down below 50%. However that is not an absolute limit if longevity is not a goal, depleting them to 20% just shortens the lifespan, it does not damage them.

That too is covered by Mark in the 12V side of life.