24v solar for Adventure Trailer

thardin68

Active member
Figuring out your peak charge rates is necessary to determine your cell balance needs. Higher rates mean more balancing current. Though if you stay out of the knee at above 98%, your balance current drops pretty quick. Nothing wrong with 24V, you just cut your wiring sizes by half, and choose components that are 24V. Lots of boats, big RVs, and small aircraft use 24V, so finding parts won't be hard.

Knowing your peak current (charge and discharge) is necessary for choosing wire sizes, inverter capacities etc.
OK, Here we delve into territory that is currently over my head. I have read some great articles discussing "the knee" and what Lithium will tolerate / prefer, but really dont understand this yet. I am determined to, however.

Of course, I assumed that once settled on some reasonable guidelines, I would need to hone in on the actual use and needs to make sure I am sizing connections and protection devices appropriately. I figured I would focus on the plan / major components first to understand the systems and then get down to the nitty gritty. Like it has been said already, might be back-asswards from an engineers POV, but it's how I think.
 

thardin68

Active member
There is really no downside to oversizing a lithium bank other than space/weight used, and some longevity loss. Lithium banks get longer lives the lower their average SOC is. SO oversizing and still charging to 100% means the bank will spend more time at high SOC. If your charging is marginal is not an issue. Down to a point, The last few percent of capacity does a bit more wear on the bank. Essentially they can be infinitely cycled in the mid range of SOC. There is one gotcha. The cells will naturally have some variance in efficiency and capacity. So every 2-10 cycles you need to get the bank into the mid 90s soc for a few minutes while charging. This allows the balancers to bring all the cells up to the same SOC. If this doesn't happen, eventually one of the less efficient cells will drift down in SOC compared to the others. So the whole bank could be at 20% SOC, but one cell is down near 5%. At which point the BMS could drop the bank out to protect that one cell (or cell group).

I don't know if you need the power to run without gen or solar for 3 days or not. Thats a call you need to make. You could always move to a camp with electrical hookups.

putting a 1800w charger on the bank makes sense. Larger works with the 3500w gen, but make sure it can be de-rated to run from a 15A oulet, as those are often available at camps with marginal hookups.


What is the capacity of this 1/2 Tesla bank you are looking at? Number of cells? How are you going to wire them? How many parallel strings?

If you aren't going to use your work computer very often, then a larger bank would be okay with 600W solar. If you are using that computer setup often, a 600W setup is going to struggle meeting your needs in non-optimal conditions.
This is comforting information and tends to support my limited knowledge / understanding. I am obviously interested in learning what the batteries like in an effort to maintain them for safety and longevety. I do understand the balancing issue and where you are going with that - I'll keep that in mind to research more. This "gotcha" is exactly what I was hunting for and a serious driver to ensure I have a charger that will allow me to charge as quickly as possible with Shore power or Generator....

Here is some info from one of the online providers of the Tesla module. It is 1/16th of the Tesla S Model battery pack. They are advertised as 85KWH batteries, but skeptics are claiming actual output as closer to 73KWH. So if simply doing math based on advertised data, the actual capacity could be a bit less. But for $500 from a tested module, I don't see how I could go wrong...

tesla-model-s-lithium-ion-18650-ev-module-22-8-volt-5-3-kwh.jpg

These used 18650 Tesla battery modules from a 85kWh Model S. These are currently the best battery on the market for energy density, allowing many classic conversions to get well over 200 miles per charge. Model S modules are comprised of 3400mAh cells arranged in a 74p6s configuration. They are rated at 500 amps, 750 amps peak. They have an integrated liquid cooling/heating system, but they can also be air cooled in light duty cycle applications. They also have an integrated connector with cell level connectivity for BMS systems and two integrated thermistors. The packs contain 444 cells, and each cell is independently safety fused on both terminals. A preferred configuration for a typical AC50 application is 1p5s for a total of 26kWh, or 2p5s for a total of 53kWh of energy with a total of 10 modules. These work particularly well in 24 volt off grid solar applications as well.

Capacity: 232Ah, 5.3kWh
Height: 3.1 Inches
Width: 11.9 Inches
Length: 26.2 Inches
Weight: 55 Pounds
Bolt Size: M8
Voltage nominal: 3.8V/Cell, 22.8V/Module
Charge voltage cut-off: 4.2V/Cell, 25.2V/Module
Discharging cut-off: 3.3V/Cell, 19.8/Module
Maximum Discharging Current (10 sec.):750 Amps
Warranty Period: One year
 

luthj

Engineer In Residence
That bank is a 6 series arrangement. You will need to monitor each parallel cell group in each string. That pack is a 74p6s arrangement with 444 cells. Each parallel cell group (74 cells) must be monitored for voltage. The built in connector will have this wiring available, as well as 2 thermistors for monitoring pack temp.

Integrating one of these is no small task, so be prepared to "do the math" It may not feel like the easiest approach, but numbers won't lie.

For reference that pack is about the same energy content as my 12V 510AH AGM bank which weighs 300lbs, and cost 1400$. Of course lead acid doesn't need a BMS with cut-out relays either.

Most any quality charger/inverter etc can be programmed for you needs. A Victron/Magnum/Outback unit is fully programmable, and you can set cut in/cut out voltages, charge time and voltages. With an integral shunt and monitor you can specific return amps for charge termination. You can configure automatic gen start if desired. You can limit charge current etc. These units will usually provide 80-120A integral chargers. Which is 1.9-2.8kw at 24V.
 

thardin68

Active member
That bank is a 6 series arrangement. You will need to monitor each parallel cell group in each string. That pack is a 74p6s arrangement with 444 cells. Each parallel cell group (74 cells) must be monitored for voltage. The built in connector will have this wiring available, as well as 2 thermistors for monitoring pack temp.

Integrating one of these is no small task, so be prepared to "do the math" It may not feel like the easiest approach, but numbers won't lie.

For reference that pack is about the same energy content as my 12V 510AH AGM bank which weighs 300lbs, and cost 1400$. Of course lead acid doesn't need a BMS with cut-out relays either.

Most any quality charger/inverter etc can be programmed for you needs. A Victron/Magnum/Outback unit is fully programmable, and you can set cut in/cut out voltages, charge time and voltages. With an integral shunt and monitor you can specific return amps for charge termination. You can configure automatic gen start if desired. You can limit charge current etc. These units will usually provide 80-120A integral chargers. Which is 1.9-2.8kw at 24V.
Yeah, that's inline with what JonyJoe101 had indicated. I understand the connectors are readily available online. I understand that the configuration will require balancing of 6 cells essentially.

There are a number of people that are claiming to use these at fractional C with very little variance in voltages. Some have claimed hundreds of cycles without ever having to balance charge.

I'm figuring I'll be a little more conservative here ad keep things in check / balanced. Back int he day, I used to charge 3S Lipos with a balance charger that would take care of it for me. How does one adjust for differences in cell voltages in something like this? I understand the EV folks prefer to bottom balance and those using for offgrid power tend to top balance. My question is, what exactly is the procedure for balancing? Would I be applying charge specifically to single groups of (74) cells? I'm assuning this is a "manual adjustment" when you see a cell getting out of line?
 

luthj

Engineer In Residence
Balancing must be fully automatic. Manual balancing is a recipe for disaster. A BMS will connect to each cell group and monitor voltages. Most common balancing approach is to shunt current around the cell (or cell group) when it reaches a certain voltage, or voltage differential from the other cells during charging. This is called top balancing. For example, one cell may reach 4.1V while the others are at 4.05V. This cells balance circuit will bypass some current around the cell. In this case, maybe 2-5A. The Telsa cells are very uniform, which means very little balancing is needed at fractional C usage.

For fractional C usage, top balancing is preferred. Adjust your discharge cut-off and charge voltages conservatively. You want to get your charge voltage just under the upper knee for longevity. This allows balancing to occur but prevents the fairly rapid aging that occurs if you spend a lot of time above 95% SOC. This reduces the packs usable capacity some, but not a problem for your application.
 

thardin68

Active member
Balancing must be fully automatic. Manual balancing is a recipe for disaster. A BMS will connect to each cell group and monitor voltages. Most common balancing approach is to shunt current around the cell (or cell group) when it reaches a certain voltage, or voltage differential from the other cells during charging. This is called top balancing. For example, one cell may reach 4.1V while the others are at 4.05V. This cells balance circuit will bypass some current around the cell. In this case, maybe 2-5A. The Telsa cells are very uniform, which means very little balancing is needed at fractional C usage.

For fractional C usage, top balancing is preferred. Adjust your discharge cut-off and charge voltages conservatively. You want to get your charge voltage just under the upper knee for longevity. This allows balancing to occur but prevents the fairly rapid aging that occurs if you spend a lot of time above 95% SOC. This reduces the packs usable capacity some, but not a problem for your application.
Understood. That is good news.

Now, how do you ensure a balanced charge automatically? I assume there is a module or component i need to add in here?
 

dwh

Tail-End Charlie
So, this unit would take advantage of the power output of the 3,500w generator (something i have not wrapped my head around yet)?
Yes.

70a@28v into the battery is around 2,000 watts, which will fully recharge the battery from dead in around 4 hours. The 2,000 watt load also pushes the gen up over 50% load, where standard gens are most efficient in terms of watts per gallon.

And it still leaves enough overhead to run the computer or an a/c unit.


Would this unit be programmable to the parameters preferred for Lithium vs more traditional Lead Acid or Gel type batteries?
Programmable is programmable. I haven't studied the manual but I'm sure it would work just fine.

You can use any charger for lead-acid (and gel and AGM are both lead-acid BTW) to charge lithium. You only have to set it to bulk to X voltage, no absorb stage, and then float at some voltage lower than the lithium's normal resting voltage.
 

luthj

Engineer In Residence
I have read some info that suggests that the factory modules have an integral BMS. I am not sure what it is capable of, or if it works standalone. That is something you will need to educate yourself on. If the built in bms is not going to meet your needs, you can buy a generic BMS unit, and wire it to the battery through the service connector.
 

thardin68

Active member
Yes.

70a@28v into the battery is around 2,000 watts, which will fully recharge the battery from dead in around 4 hours. The 2,000 watt load also pushes the gen up over 50% load, where standard gens are most efficient in terms of watts per gallon.

And it still leaves enough overhead to run the computer or an a/c unit.




Programmable is programmable. I haven't studied the manual but I'm sure it would work just fine.

You can use any charger for lead-acid (and gel and AGM are both lead-acid BTW) to charge lithium. You only have to set it to bulk to X voltage, no absorb stage, and then float at some voltage lower than the lithium's normal resting voltage.
excellent, thanks !
 

thardin68

Active member
I have read some info that suggests that the factory modules have an integral BMS. I am not sure what it is capable of, or if it works standalone. That is something you will need to educate yourself on. If the built in bms is not going to meet your needs, you can buy a generic BMS unit, and wire it to the battery through the service connector.
I will, thanks. I'll report back..

From what I have seen, most add a bms as the factory unit would require a ton of work to utilize.
 

thardin68

Active member
Did some research. Monitoring and balancing these appear to be easy and cheap. HSR Motors can provide a custom converter board to replace the poprietary unit and a Tenergy 5 in 1 cell meter / balancer for next to nothing.

I also see that Xantrex offers a charger inverter 807-2055 which is a 2000w pure sinewave inverter with a 55amp charger. Aside from 15 amps less on charger, wonder what else I'm giving up for 1/2 the cost of the Samlex unit... EDIT - Scratch that, it's only 12V. Most seem to be in the same ballpark. Is AIMS an inferior product? they seem to be a bit lower in the price range for similar specs.
 
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thardin68

Active member
OK, Spent some time looking at the Inverter / Charger options. Any of the inverter Chargers that fall into a lower price point have significantly lower charging capabilities. Makes sense.

Now, Samlex makes 2 units that I would consider, the EVO 2224 and the EVO 4024. Both are the same size, include the same functionality. The 2224 offers a 2,200w PSW inverter and 70A charging capacity. The 4024 offers 4,000w PSW inverter and 110A charging capability. For 15% more money I can significantly reduce my charging times. Doubtful I would ever need the additional inverter capacity. Obviously the conductors, connectors and protection costs will increase, but these will be very short runs.

With the larger unit I could potentially Max out my generators capabilities and limit the time required to charge. I understand I can program the charger to find the sweet spot for the generator to avoid being too hard on it.

I also understand the Samlex unit has the ability to take the output from the solar charge controller and manage that as well. I have downloaded the manual which is extremely informative, and will be taking some time to go through that to ensure it has the ability to program all of the parameters I will need to adjust for the Tesla Module.
 

dwh

Tail-End Charlie
So you could run 4x50a@24v battery chargers
Oops, brainfart - I was somehow thinking of 12v instead of 24v.

That should say 2x50a@24v (or 4x50a@12v).


(Kinda surprised none of the other electronheads around called me out om that goof. :) .)
 

thardin68

Active member
get a 100ft 10awg extension cord from HF and its far enough away that even in a tent it will be far more tolerable, stuff it behind a rock or tree and it makes it real quiet.. you already got the genny, put it to use.. I recently converted from tent camping and camped at tons of sites and almost never found a camp that didnt allow genny use in the day time.. yeah they all restrict hours of operation and some may require them to be the quieter ones but outright genny bans are fairly rare outside national parks.. now that you got a trailer those tent-only sites wont be welcoming you anyhow and most camps that allow trailers will allow gennys.. Ive even had night time bans lifted because sub freezing temps were going to run everyone's furnaces all night and most people were gonna wake up with a dead battery w/out electricity.

If your in national forest land you can camp anywhere you want, no quiet time.. run it all night long if you want, if there is anyone nearby the'll be several hundred yards down the road and wont hear a thing.. plenty of room to take that extension cord to its limit.. however having data signal for work is going to be the harder part, if your planning on working your going to need to stick somewhat close to civilization.

I'd also look into a newer computer, a new i5-9600K complete system can draw less than 50w idle, less than 100W w/single threaded tasks, and less than 150W fully loaded.. and it outperforms most previous generation i7 CPU's.. monitor power also goes up exponentially with size, so go with the smallest size you can tolerate.. yeah a 30in might be great, but a 22in might be half the power load and far more practical.. Solid state drives use virtually no power compared to spinning disks, so thats another thing to consider.

and I agree, you needa determine your base loads before you start throwing a ton of money into this or you could end up in a terrible position.. Calculators are great and all but nothign beats real world testing, get your self a watt-o-meter, both AC and DC versions.. run each appliance for 24h in typical environmental conditions and figure out how much power they draw a day, then add up all your appliances and that is your minimum budget..

yeah that tesla battery is gona be cheap, but how much is a 24v charger adequate for it gonna cost (I got a 630w AC charger for my 100A Lithium, your gonna want a few thousand watts to quickly charge that thing)? you should also be looking past 24v solar.. Panasonic HIT is what i'mna be using its like 65v, the MPPT controller will gimme the charge voltage I want.. Also hooking up a undersized solar and undersized AC charger up to a very large bank is totally ass backwards and leaving much of the Lipo benefits on the table when it takes several days of no usage to get it back up to full.. its a much better position to be in with oversized charging sources for your battery bank, especially solar since you almost never get its full rated outputs.. if 650W of Solar and/or AC is the sweet spot for my single 100AH battleborn, neither are going to be near adaquate for a lithium system several times its size/capacity... If you do really need that capacity, you may end up using that generator far far more than you ever wanted since your never going to get enough power to keep the Tesla batteries in good shape otherwise.. then your right back where you started, just with alot less money and room.

When I jumped to Lithium Batteries, the capacity I needed actually went down in the real world.. I needed far more capacity with lead weights because the batteries charged so slowly, charge/discharge effencies were terrible and variable with larger loads.. with lead 24h at 10A does not equal 12h at 20A, you'd be lucky to get 8h.. then a vast majority of the capacity was untouchable or else id risk junking the batteries.. I was planning for >300AH of lead and concerned I may need more still, but with Lithium 100AH is quite sufficient for me.. since I can charge it up in a few short hours, instead of all damn day it means I dont need as much capacity as I would otherwise, now running generator daily for a couple hours is no big deal, and there is no pressure on me to get it back to 100% SOC.. its far better than lead where you gotta run it all day because its gotta soak in absorb mode and the things dont last if you let leave em partially charged for days on end.
Dreadlocks - I have been intrigued by the Panasonic HIT panels and am trying to understand the benefits / costs. I see that I can get (2) 330w panels with the MPPT optimizer (transformer) for about $800.

Does the use of these panels with the optimizers eliminate the need for a dedicated solar charge controller (or do they simply get the non standard voltage in line to work with the charge controller you still need to have)? I'm looking at Charger / Inverters that allow me to run Solar Charge Controller imput through them. I have not figured out what parameters you can control through the Inverter / charger with regard to the PV input, but I am curious if I am able to use the MPPT optimizers as my solar charge controller(s), eliminating the need for another component in the trailer.

I'm attracted to the efficiency and ability to work with some shading and better performance off angle. With your knowledge, can you provide a newbie with a comparison between these and say (2) Renogy 300w Mono panels that dont need the Optimizers (which can be had for about 20% less)?
 

dwh

Tail-End Charlie
110a x 28v = 3,080 watts.

Theoretically, that's within the capabilities of your generator. But...there is also power factor to consider.

https://en.m.wikipedia.org/wiki/Power_factor

If the charger has a power factor of 1, then it will draw 3,080w from the gen to supply 3,080w to the battery. But if it has a power factor of 0.8, it will draw 20% more from the gen to supply the same 3,080w to the battery...which would exceed the 3,500w continuous load rating of the gen.

Of course, the Samlex can be programmed not to exceed the limit of the gen. But that is going to reduce the amps to the battery. If that reduction is around 20%, then you are only going to shave 1 hour off your charge time.

Personally, I would choose to save the 15% cost since you don't need a 4,000w inverter. Charging at 70a and still having 1,500w available overhead to run computer and/or a/c would be my choice.
 
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