I’ve finally pulled the trigger. I ordered my battery bank. Earlier in the week I also ordered a BMS for it. Both should be here in a month.
Meanwhile, a buddy pointed out a flaw in my upgrade. My new panels are about 60 volts. I was thinking 48v pack with 60v solar cells… I should be fine. If that was all there was to it, I would be. However there is more to it.
First, I forgot to take into account the charging vs nominal voltage of the chemistry to which I’m using. LiFePo4 cells have a nominal voltage of 3.2v. Infact, most of their power is in the range of 3.2 to 3.3 volts. For my “48v” pack, I’ll be using 16 cells in series. This will add the voltages of all 16 battery cells together. With all of them aded up I’d be looking at a “nominal” pack voltage of 51.2 or 52.8.
Still no so bad, my solar panels are rated at 60 volts, we should be good. My Victron 150/100 MPPT controller does have a 5 volt requirement above the battery voltage to start charging though, but it only takes 1 volt above it to keep going. So far, so good, but whoah… not so fast. Lets look at a typical real world voltage right now.
This screenshot is taken at about 10AM. A pretty good time to be in the sun for the panels. The sun is high enough overhead to get good direct light where I am, but its not yet too warm. 57.2 Volts is all that I’m getting. That takes a bit of my breathing room away, but if the batteries were low they would still start charging. However this is the “ideal” time. We don’t always have ideal time. But wait, there is more
Per cell the charge voltage can go up to 3.65 as per LiFePo4 documentation (Although I’ll likely go lower for longer life) That is 58.4V to charge the pack. Remember, I need 5 volts on top of that to actually allow the charging to start, and 1 volt higher in order to keep charging. The end result here is that, if it ever stopped charging it wouldn’t start again, even if the batteries were not full. At least, not until they got low again.
The solution is normally to double up the panels. Instead of 60 volts, I’ll have 120 volts. That seems like a reasonable deal, and my solar charge controller can take a PV array up to 150 volts. Except that, when it gets cold or hot it changes the voltage of the panels. The panels produce more volts when it is cold out, and less when it is hot. This is why you can actually get more power from panels when they are cold assuming the same quantity of light!
So, I had to do a bit of math. Solar panels are rated at 25C. My panels voltage will change by -0.35 for each additional C. I made a quick spreadsheet and quickly discovered that with two panels in series I could only get to -2C and stay within the charge controllers rating. -3C and the panels could produce enough power to damage my charge controller. 28.4F and warmer is fine, but anything colder then that when the sun is hitting my panels could break my controller. I’ll need to upgrade that before winter, or I risk damaging a very expensive solar charge controller or never fully charging my battery bank.
My current controller costs about $750, and it does 150 volts, the next one higher in voltage does 250 volts and costs about $950. This solar charge controller would be good down to -146C or -230.8F. If my van gets that cold, I’ve got bigger problems to worry about. So the problem isn’t that it can’t be solved. The problem is I just upgraded to this charge controller 2 weeks ago!
The spending spree continues… As I mentioned today I just spent another thousand on the batteries (Of which I likely need another set, maybe 2) Additionally, with my few checks I’ll need to spend similarly to finish this project the way I’d like. Specifically I plan on purchasing the following
- 250/100 Victron MPPT
- 12V to 48V DC to DC Battery charge from Sterling Power.
- 48V to 12V DC to DC Battery charger, OR power supply.
- 48V Inverter
- I’m seriously considering adding an EV charge port, and a 48V charger for that as well. Additionally being able to plug into shore power.
While this power upgrade project is going to get even more expensive then I had anticipated it will provide me with the most options moving forward. In looking at what equipment is available, and how we are moving more and more towards EV charging stations being easily accessible it is very likely that 48V will be the defacto standard for house batteries in vans and RVs in the next decade. As technology keeps improving and marching forward I suspect it will become very easy to charge up the house batteries.
With the long life of LiFePo4 battery packs, I’ll likely still be running the same pack I buy this year. Meanwhile, I’m broke and I literally just got paid hours ago. As I see it my next several checks will likely have the same fate, it just really hurts that saving $200 is going to cost me so much more. Mistakes were made, but so was progress.…