VFX3648 + LiPo Power for Food Truck

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VFX3648 + LiPo Power for Food Truck

Postby dotkickpra1 on Tue Mar 19, 2019 2:39 am

We're starting a process to convert a food truck from a generator to battery power. We've obtained 4 Bosch lithium ion (LiMn2o4 to be specific) modules/batteries, which utilize 6 Samsung lithium ion cells per module. These modules are the same batteries used in the Fiat 500e and Mercedes Benz b250e cars. They're super stable, have excellent thermal stability and a long life span.

I have 4 modules. Each module is 24vdc and 68Ah of capacity. Each module is wired into a series pair for 48vdc and 68Ah of capacity. Then each pair is wired in parallel for a total power pack of 48vdc and 136Ah of capacity. I've done an amp hour study of the equipment in the food truck, and the power pack should be able to run the truck for 7 or 8 hours even if everything is running at 100% capacity, which is very unlikely to happen. In the real world, we should be able to easily do a 4 hour lunch, go back to our commissary kitchen to restock the truck, do the dishes & recharge the cells. Then we'll head out for another 3-4 hour dinner or late night service before going back to the kitchen for the night.

Since we have a 48 volt system, our choice for an Outback inverter/charger are limited to only a couple of choices. I've found a couple of different inverters on eBay - a VFX3048 and a VFX3648 - and they seem like the logical choice.

My only question, of course, is the brand new type of LiPo batteries we're using. Does anyone know if the charge algorithms of VFX3648 can accommodate newer batteries like the ones we have? Can an older unit be updated to newer battery algorithms? We already have the batteries in hand, so we basically HAVE to match a charger to the batteries.
Thanks.
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Re: VFX3648 + LiPo Power for Food Truck

Postby Kurt Lundquist on Tue Mar 19, 2019 8:10 am

What is the operating voltage range of the battery pack?

What charge voltage(s) do you need?

-Kurt
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Re: VFX3648 + LiPo Power for Food Truck

Postby Jaliance on Sat May 11, 2019 10:48 am

Any update for this?

I have a similar project 2 24 Volt Tesla battery packs in
Series requiring 49 volts absorb and a slightly lower float voltage.

I am currently using a VFX3648 which seems to be limited to an absorbe charge voltage of 52 volts.

Can anyone tell me if it is possible to program the FX to charge the batteries at a voltage less than 52 volts or is this a hardware limitation?

Thanks
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Re: VFX3648 + LiPo Power for Food Truck

Postby Kurt Lundquist on Mon May 13, 2019 8:15 am

52V is lowest Absorb voltage for the VFX3648 without temperature compensation. If the RTS thought the battery bank was 45°C it would target ~2.4V lower which would allow you to charge below 50V. You would need to wire in a resistor in place of the RTS. I have no idea what the resistance value should be for 45°C. I guess you could put the RTS in a 45°C cup of water then measure Ohms to find the proper resistance. Considering the cost of your battery bank it probably doesn't make much sense to band-aid the inverter for the application. The newer style VFXR3648A is the proper solution for Lithium. It offers a much wider range of charging options. I suggest you sell the VFX and get the newer style.

If you have the OutBack FN-DC you can use it to shut off the inverters to protect the battery bank from discharging too much. See page 6 of this app note http://www.outbackpower.com/downloads/documents/appnotes/simpliphi_app_note.pdf

-Kurt
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Re: VFX3648 + LiPo Power for Food Truck

Postby Jaliance on Mon May 13, 2019 2:05 pm

Hello Kurt thank you for your detailed response. I am considering your suggestion to sell my VFX and upgrade to the new VFXR however I need to eliminate my less expensive options.


In regards to modifying the RTS I would essentially be tricking the vfx to provide the desired voltage

- Would it be possible for me to find and provide RTS resistance for 50°C rather than 45°C? If so this would target the desired voltage of 49V
- Aside from the fact that this would be a less than ideal solution and I would essentially be hacking the RTS forcing the VFX to provide the desired voltage...Are there any other reasons why this would be a bad idea?

In addition to the VFX3648 I have a FM60 solar charge controller which would be the primary charge source to the batteries.

Lastly I have a Victron BMS 712 with temperature protection and automatic disconnect as well as a Victron 48-100 battery protect with programmable over voltage under voltage disconnect.


Thanks in advance for your time
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Re: VFX3648 + LiPo Power for Food Truck

Postby raysun on Mon May 13, 2019 11:29 pm

Kurt can give you a more nuanced answer certainly, but here's a few things to consider:

• The RTS hack is clever but not without some challenges that will need to be addressed. The RTS is essentially a temperature variable resistor. Replacing it with a fixed value resistor will require one that does not vary with temperature, and most all resistors will by a factor called TCR (Temperature Coefficient of Resistance.) You are going to need to find a precision resistor with as low and stable a TCR as possible. More likely a variable resistor will need to be added as well so the proper value can be adjusted by 'trimming the pot'.

The hack will leave the batteries at risk in the event the resistance drifts, or fails in another way. Some form of fail safe, like the Victron perhaps, to cut off charge in the event of overvoltage will be needed. The cutoff circuit will need to be able to handle 'worst case' charging current, so look to be adding some relay switching to the circuit.

If it were I doing this, I'd look for a set of old lead acid batteries to use to work the bugs out. Shiny new Lithium batteries going up in smoke, or worse flames, would impact my normally sunny disposition. Other folks may be more adventurous.

• The FlexMax 60 charge controller can follow the temperature compensated charging curve of the inverter if both are plugged into a Mate via a Hub. A couple more pieces of kit to get if not already on hand.
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Re: VFX3648 + LiPo Power for Food Truck

Postby jbakker on Tue May 14, 2019 4:25 am

I think I read it here or someplace that you will ruin your first set of batteries before you learn how they work and how to work them.
Been there and done that.
So I agree with trying to find a set of lead acid batteries to play with.
On the other hand is there any way that you can physically rewire the packs from 6 cells to 7 cells per pack?
This will give you 28 volts. 28 + 28 = 56 and that makes the charging a lot easier.
Assuming that you charge the cells to 4 volts each, would be an excellent idea to think about a BMS as a safety, will limit the charge and discharge of your pack, safety on them is very important!
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Re: VFX3648 + LiPo Power for Food Truck

Postby raysun on Tue May 14, 2019 6:56 am

I like the notion of bringing the battery in line with the natural charging characteristics of the equipment, if at all possible, though probably not practical with pre-packaged battery modules.

Murphy's law of system design dictates "Any system dependent on a fail-safe, will fail due to failure of the fail-safe."

If these batteries are expensive, I'd be inclined to deploy the kit that is designed to properly charge them from the start.
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Re: VFX3648 + LiPo Power for Food Truck

Postby Kurt Lundquist on Tue May 14, 2019 10:15 am

Jaliance wrote:- Would it be possible for me to find and provide RTS resistance for 50°C rather than 45°C? If so this would target the desired voltage of 49V


2.4V is the max compensation so going past 45°C isn't going to make a difference. If you need <49.6V this will not work.

Jaliance wrote:- Aside from the fact that this would be a less than ideal solution and I would essentially be hacking the RTS forcing the VFX to provide the desired voltage...Are there any other reasons why this would be a bad idea?


If the FM charge controller sees 50°C for the battery bank it will stop charging and display a "Batt Too Hot" error message. There could be a host of other possible issues. The other posts have brought up some good concerns.

You could purchase a dedicated battery charger to use instead of the VFX. Space on the truck is probably a premium though and finding something that can be set to charge at 49V might be difficult.

What type of food will the truck be serving up?

-Kurt
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Re: VFX3648 + LiPo Power for Food Truck

Postby Jaliance on Wed May 15, 2019 2:10 pm

Thank you everyone for your valuable input at this point I have decided it would be best to sell my VFX3648 and upgrade to the VFXR3648A.

Although the RTS hack seemed promising it is limited to 2.4 volt compensation which is still .6 volts above my targeted voltage eliminating it as an option.

I have come to agree with the general consensus that it would be best to have all the equipment operating within it's designed spec.

The newer VFXR3648A has Advanced Battery Charging capability with expanded charge voltage and time perameters which justify the upgrade expense.

Kurt This system will be used on an off grid cabin.

Thanks
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