Considering doing a 24v setup.

[KingArthur]

So here are my thoughts...

I've had the recent luck lately via craigslist of coming into ownership of both an Engel fridge and a nice inverter (that I forget the name of at the moment) that both support a 24v DC connection. Right now I've purchased a few small doodads of Amazon, such as LED lights, that are 12v, but these things could be easily returned and swapped out for 24v DC items that are similar. My solar controller also supports a 24v output. 

I suppose the only thing that wouldn't be compatible at this point would be a connection to the vehicle battery. I doubt converting my Astro to a 24v system would be something that I'd want to do. I wasn't really planning on using the vehicle battery anyway, so I supppose it isn't really a big deal.

One reason I'm thinking about doing it in the first place is because I plan to build a house battery out of lithium 18650 cells. Right now I totally LUCKED OUT and got about 300ah worth of marine grade lead acid batteries for free, so I'll be using them for the foreseeable future until I get my lithium pack made. The lead acids are vented and aren't all similar capacity so I'm going to want a better option down the line anyway.

With a 24v system, making a lithium pack out of 18650 cells becomes a bit easier. Because of lithium cells being 3.7v, I cannot use four of them in series because at full charge (4.2v per cell), the voltage would be much too high for anything expecting 12v DC. That means I have to use three of them in series, which would give me 12.6 full charge and 11.1v nominal, and 9v at safe empty. Most 12v DC things will not run below 10.5v DC. Sparing you the details of the math, that means that my usable capacity of any battery arranged in this way would be about 83% of whatever the actual useable capacity of the lithium pack would be. So here's where 24v comes in...

The voltage tolerance range of my inverter, for instance, goes from 10.5v-15v at 12v DC to 21v-30v DC at the nominal 24v setup. This would mean I would be able to use the entirety of my lithium battery's capacity, being that a 24v pack would mean having seven 18650 cells in series, giving me the voltages of 21, 25.9, and 29.4 respectively. 

So I haven't checked my fridge yet or anything else, but I'm sure that it's all capable of that similar range. It seems to be a standard. Of course I would check before I would go and build it, but at this point it's looking like a pretty attractive choice.

Last advantage is the efficiency of the solar charge controller. It appears that the effectiveness of MPPT charging goes up the closer the beginning and ending voltages are to eachother. For instance, if your solar array is outputting 60v at say 6 amps and your charge controller is set to charge at 12 volts, it will charge at 80% efficiency. If the beginning voltage is 30v instead, it will have a higher efficiency. This information I got out of the pdf manual of the Renogy Tracer. This being true, my panels could be wired in parallel, giving me 37v output and converting to 24v instead of 12v. 

Any other thoughts regarding this scenario or things I didn't think of?

 

[frater secessus]

I would have thought greater voltage differential would increase MPPT efficiency. Hopefully someone will step in and break it down for us.

 

[BradKW]

I'm building my 24v bank as we speak, so will comment more when not using phone. Please link the inverter, what you believe you have is unusual.
Most of the diy lithiums you Google or YouTube are frightening fire hazards that burn down RV's...

 

[Trebor English]

You have lead acid batteries now, you want 18650 cells. Are these AA size? By the time your lead acids are done maybe you might want LiFe lithium iron. That chemistry has distinct advantages.

 

[frater secessus]

Please link the inverter, what you believe you have is unusual.

Plenty of 24v inverters on amazon.

 

[BradKW]

He's saying that he has one that will operate on both 12v and 24v... I've not seen this before.

 

[KingArthur]

I'm building my 24v bank as we speak, so will comment more when not using phone. Please link the inverter, what you believe you have is unusual.
Most of the diy lithiums you Google or YouTube are frightening fire hazards that burn down RV's...

Here's the inverter: (obligatory dirty fingernail)



As for the fire hazard bit... Lithium ion batteries are some of the safest batteries that have ever been made. I don't mean to insult you but I hear the fire thing a lot, and I think the level of fear that people have for the technology is unwarranted. Of course building your own pack can be hazardous but just like anything else if you know what you're doing your chances of failure decrease. Quite simply I'm willing to take the risk with 18650's. One major advantage they have over any other battery technology is that they are modular. I can add or subtract banks of these guys at any time without any sort of major overhaul or expenditure.

 

[KingArthur]

You have lead acid batteries now, you want 18650 cells.  Are these AA size? By the time your lead acids are done maybe you might want LiFe lithium iron.  That chemistry has distinct advantages.


You're probably right but the price is extraordinary for those guys. I've already got the 18650's and I've spent much less on them than even AGM lead acids. 

 

[johnny b]

Since you seem to have some experience with these little cells, How do you do the connections? I've seen small, purpose built spot welders for building battery packs for RC stuff...am I on the right track here?

 

[BradKW]

Here's the inverter: (obligatory dirty fingernail)



As for the fire hazard bit... Lithium ion batteries are some of the safest batteries that have ever been made. I don't mean to insult you but I hear the fire thing a lot, and I think the level of fear that people have for the technology is unwarranted. Of course building your own pack can be hazardous but just like anything else if you know what you're doing your chances of failure decrease. Quite simply I'm willing to take the risk with 18650's. One major advantage they have over any other battery technology is that they are modular. I can add or subtract banks of these guys at any time without any sort of major overhaul or expenditure.

I'm pretty sure that what you are looking at in the picture is actually the specs for two different units: a 12v 1500 watt, and a 24v 3000 watt. If you look at the top of the page there, you'll see that the manual is for both models. I agree it's detailed in a confusing way, but I guess they're operating under assumption that you know there's no such thing as an inverter that can take either 12v or 24v inputs. Hook up 24v to that 1500 unit and you might have smoke. Something to keep in mind if you decide to play with the more volatile and dense Li-ion cells. What were you planning to do for a BMS?

In general there's a few reasons to go 24v instead of 12v. My primary reason was because I have 1000 watts of solar, and even the 60 amp Morningstar doesn't want that...but step up to 24v and all of a sudden there's tons of headway. 
Another reason is cable size...24v requires much smaller cables than 12v, a savings plus easier. In a small vehicle though, this isn't a big factor.
There is a minor efficiency improvement with 24v to 110v inverters...nothing to dance about, and in fact you can negate it by having to use step downs for some 12v only devices.

If you had a $3000 Magnum or Morningstar 24v inverter, I'd say hmm, maybe go for it. But with current devices and loss of alternator charging, really no reason for it I think.

edit: that manual may also be showing 12 and 24v flavors of both the 1500 and 3000 models. Somewhere on your unit should be a serial number or something you can google to be sure what you have before powering it up...

 

[Trebor English]

Packs of 3.5 cells is obviously not a good idea. Seven cell packs sounds like it could work. Since you have 18650 cells, an inverter, an MPPT controller, and a fridge that can take 24 Volts this sounds like a good idea. Although it is odd it sounds like it should work without buying a bunch of more stuff. The pounds per Watt hour and cubic feet per Watt hour should be better than lead acid.

I don't think the efficiency difference between different voltages in and out of the MPPT controller will be significant. The difficulty of charging AGMs at high enough rates or equalizing flooded cells would probably be more noticeable.

Can you connect two MPPT controllers to one solar panel array? If that can work it might be possible to have a 12 Volt lead acid bank and a 24 Volt lithium bank. This could take care of some 12 Volt loads that aren't easily adaptable to 24 Volt or allow the occasional charging of the chassis battery. There is always going to be that latest RoadPro toy that is only available in 12 Volt.

 

[blars]

If you want safe Lithium batteries, go with LiFePO4. Note that Boeing had a fire related to one of the older Li chemistries. You'll need lots of 18650's, I think prismatic is more practical. LiFePO4 should be charged at most at 3.5v/cell, full charge is about 3.3v/cell. So 4 cell packs make nice "12v" replacements, or 8 for 24v.

 

[Trebor English]

The fire issue is a news issue. Both Boeing battery failures lead the national news several days. Both, all two of them. There was a hoverboard fire in Orlando that was on the tv news for a week. OMG you would think the battery jumped out of the toy, ate the child and its baby brother and then burst into flames like the Hindenburg. The child was not injured. The cause of the fire was overcharging. The damage was limited to the destruction of the toy. If you charge and discharge within their limits they don't have the thermal runaway failure mode.

Do you realize how disfiguring sulfuric acid can be to a face? A spalsh to the eyes can be blinding. Eating the lead in just one car battery can ruin your kidneys. Lead acid is horrible. Treat the technology with the respect it demands and you can use it safely to make your life better. Believe the hype on the tv news and you can spend the rest of your days cowering in fear.

Sure, LiFe is less susceptible to thermal runaway and I have seen the video where they shoot it with a pistol. If you don't have any LiFes and do have lots of 18650s and do have a plan to avoid igniting them it seems that staying home and doing without is a less attractive option.

 

[KingArthur]

I'm pretty sure that what you are looking at in the picture is actually the specs for two different units: a 12v 1500 watt, and a 24v 3000 watt. If you look at the top of the page there, you'll see that the manual is for both models. I agree it's detailed in a confusing way, but I guess they're operating under assumption that you know there's no such thing as an inverter that can take either 12v or 24v inputs. Hook up 24v to that 1500 unit and you might have smoke. Something to keep in mind if you decide to play with the more volatile and dense Li-ion cells. What were you planning to do for a BMS?

In general there's a few reasons to go 24v instead of 12v. My primary reason was because I have 1000 watts of solar, and even the 60 amp Morningstar doesn't want that...but step up to 24v and all of a sudden there's tons of headway. 
Another reason is cable size...24v requires much smaller cables than 12v, a savings plus easier. In a small vehicle though, this isn't a big factor.
There is a minor efficiency improvement with 24v to 110v inverters...nothing to dance about, and in fact you can negate it by having to use step downs for some 12v only devices.

If you had a $3000 Magnum or Morningstar 24v inverter, I'd say hmm, maybe go for it. But with current devices and loss of alternator charging, really no reason for it I think.

edit: that manual may also be showing 12 and 24v flavors of both the 1500 and 3000 models. Somewhere on your unit should be a serial number or something you can google to be sure what you have before powering it up...

Upon looking further I think you're right... There's no indication that there was ever a 24v model made, and I can't even find on online. However a little further down on the page it does list the 24v edition being ever so slightly heavier than the 12v. So I guess case closed on that one!

As for switching to 24v for the fridge and other things... The fridge hardly draws any power at all anyway, and I would get a slight efficiency improvement from the charge controller, but I hardly think it's worth it if the inverter isn't on board. I'l just deal with the smaller capacity at 12v for the lithium cells. I'm sure ill have more watt hours per pound even considering that.

 

[KingArthur]

Packs of 3.5 cells is obviously not a good idea.  Seven cell packs sounds like it could work.  Since you have 18650 cells, an inverter, an MPPT controller, and a fridge that can take 24 Volts this sounds like a good idea.   Although it is odd it sounds like it should work without buying a bunch of more stuff.  The pounds per Watt hour and cubic feet per Watt hour should be better than lead acid.

I don't think the efficiency difference between different voltages in and out of the MPPT controller will be significant.  The difficulty of charging AGMs at high enough rates or equalizing flooded cells would probably be more noticeable.

Can you connect two MPPT controllers to one solar panel array?  If that can work it might be possible to have a 12 Volt lead acid bank and a 24 Volt lithium bank.  This could take care of some 12 Volt loads that aren't easily adaptable to 24 Volt or allow the occasional charging of the chassis battery.  There is always going to be that latest RoadPro toy that is only available in 12 Volt.

Here's more details about the struggle with 18650 cells according to what I've researched.

The first problem is that 18650 cells have a different range of useful voltage than lead acid batteries. A typical 18650 cell has about 2000 mAh of capacity at 3.7v nominal. Upon charging up one of these batteries to full capacity it will show a voltage for 4.2v. When the usable  capacity is depleted ( ie. not running the battery down to such a low voltage that it damages the battery) it will show a voltage of 3v. 

Arranging three 18650 cells in series will increase the voltage, but the capacity will remain the same, just like any other battery. So just having three of these batteries as my hypothetical bank would give me a battery with 2 Ah capacity at 12.6v full, 11.1v nominal, and 9v depleted. If I were to use 4 cells instead, the voltage would read 16.8v fully charged, 14.8 nominal, and 12v depleted. This range is too high for my inverter, and I would also expect my fridge and anything else that is designed to run on 12v nominal power. 

Obviously I'm going to need more than three of these guys to make a bank, so at this point I've purchased about 75 used laptop batteries off of ebay and I'll be making a bank of 324 cells. The bank will be made up of modules that are 3s12p, or 32 cells. After what I assume will be weeks of testing and building this thing, I'll have my 648 Ah battery at 11.1v nominal.

So that creates a second problem. The low voltage point of this pack will be 9v. Most 12v DC appliances will not run at any voltage below 10, sometimes even 10.5. According to my very possibly wrong math, that means that I'm going to lose about 17% of my total capacity because of voltage being too low. Some things might continue to work down to 9v, some might not. I'm going to plan around nothing working below 10.5v

Even with this 'tax' the watts to weight ratio and watts to size ratio and the dollar to watt ratio are all still better than lead acid, and if you're insane like me and don't mind spending 100 hours buiding a giant battery, than it's probably a good choice![/quote]

 

[KingArthur]

The fire issue is a news issue.  Both Boeing battery failures lead the national news several days.  Both, all two of them.  There was a hoverboard fire in Orlando that was on the tv news for a week.  OMG you would think the battery jumped out of the toy, ate the child and its baby brother and then burst into flames like the Hindenburg.  The child was not injured.  The cause of the fire was overcharging.  The damage was limited to the destruction of the toy.  If you charge and discharge within their limits they don't have the thermal runaway failure mode.

Do you realize how disfiguring sulfuric acid can be to a face?  A spalsh to the eyes can be blinding.  Eating the lead in just one car battery can ruin your kidneys.  Lead acid is horrible.  Treat the technology with the respect it demands and you can use it safely to make your life better.  Believe the hype on the tv news and you can spend the rest of your days cowering in fear.  

Sure, LiFe is less susceptible to thermal runaway and I have seen the video where they shoot it with a pistol.  If you don't have any LiFes and do have lots of 18650s and do have a plan to avoid igniting them it seems that staying home and doing without is a less attractive option.

Hahaha, my sentiments exactly.

 

[KingArthur]

Since you seem to have some experience with these little cells, How do you do the connections? I've seen small, purpose built spot welders for building battery packs for RC stuff...am I on the right track here?

I've been thinking about a spot welder... They aren't too expensive for the scope of project I'm getting into. I might even pay less for a spot welder than I would in the tons of solder I would end up using. And I'm sure my lungs would thank me. And I could sell it when I'm done...

Hmm....

 

[SternWake]

I have about 20 18650 extracted from laptop battery packs. I use them in a nitecore hc50 headlamp. And as a single cell USB source.

They have very little usable capacity left of their original 2200 mah.

Warm 0ff the charger at 4.2v, they can't support 'turbo' mode on the light, and can only power the usb source charging my phone for a few seconds before they cant support 0.89 amps.

Compared to my panasonic ncr18650 b. They are a joke.

I was hoping to use the laptop extractions to reduce the cycling of the pannys. But super short run times on the headlamp and inability to charge phone make them near worthless.

They can power my Bluetooth speaker at a max of 0.40amps. So i use them for reducing the cycles on the bt speaker's battery. But even them i have to swap themnout every hours or so.

i check their heat on the charger too. About 6 cells got so hot i threw them in a sandpile like a grenade. But apparently got them in time.

If your laptop cells are not lg sanyo or panasonic i would jot spend any effort making this battery pack. Even then thes cells are not matched and will develop different voltages. Causing imbalances which could end with a.fire like all those chinese hoverboards.

 

[ZoNiE]

So all the practical experience (not a lot) and research, and what people in the battery business tell me, you have to spot weld. Using a soldering iron and heating the battery is detrimental to the battery as it takes a lot of heat to solder to them properly without having cold solder joints. Also, if you don't meticulously clean the remaining flux off of the joints with a brush and alcohol, corrosion will set in. Bottom line, Ya gotta spot weld them. Spike of heat for a very short time and no damage to the cell.

I've soldered to them in the past, in a pinch, to get some "thing" on my bench to work, but this was with batteries I didn't care about.

I think the Tesla Pack uses these cells, does it not? Perhaps only the original roadster model did.

Using used batteries from different sources may give you lackluster results as some batteries will be near end of life and others will not.

LiFePo4 is the way to go.

I have a 24V inverter that a friend gave me. He bought a pallet of them at an auction. It is a Pre-Magnum Dimensions Sensata MIL-24-2200, very popular with the military and utility trucks. It is the perfect wattage for me, 2,200W but I have a 12V RV and really not planning to go 24V for a number of reasons. Perhaps I'll use it for a Cabin someday, but then I'll want a bigger one!!!

It's tempting to convert the RV to 24V, as the only remaining 12V loads would be the lighting, the furnace Fan (which I could mod) and the water heater and Heat pump controls which could all be powered by 24-12V DC-DC converters.

 

[KingArthur]

I have about 20 18650  extracted from laptop battery packs.  I use them in a nitecore hc50 headlamp. And as a single cell USB source.

They have very little usable capacity left of their original 2200 mah.

Warm 0ff the charger at 4.2v,  they can't support 'turbo' mode on the light, and can only power the usb source charging my phone for a few seconds before they cant support 0.89 amps.

Compared to my panasonic ncr18650 b. They are a joke.

I was hoping to use the laptop extractions to reduce the cycling of the pannys. But super short run times on the headlamp and inability to charge phone make them near worthless.

They can power my Bluetooth  speaker at a max of 0.40amps. So i use them for reducing the cycles on the bt speaker's battery. But even them i have to swap themnout every  hours or so.

i check their heat on the charger too.  About 6 cells got  so hot i threw them in a sandpile like a grenade. But apparently  got them in time.

If your laptop cells are not lg sanyo or panasonic i would jot spend any effort making this battery pack.  Even then thes cells are not matched and will develop different  voltages. Causing imbalances which could end with a.fire like all those chinese hoverboards.

Sounds like some seriously bad luck. Even no-names shouldn't behave that way. My guess is that your 18650's are either very old or were mishandled in the past. I'm assuming I'll get about 70% of the batteries out of the laptop packs that have an acceptable capacity remaining. Anything below 1500 mah and anything that gets warm under a slow charge won't be used. If I need to get more laptop batteries, so be it![/quote]