Need Your thoughts... risk of loosing out of power at low sun exposure 12V, 24V vs 48V battery system with limited solar array.

[mr2web]

Hi!
I did build a solar array on the roof of my RV last summer. This solar array including the solar charger was scaled to be able to provide for both a 12V battery system as well as for a 48V battery system. Naturally it would also fit a 24V battery system as well.

My initial plan was to run the old 12V GEL battery until it gave up or I decided that I needed more power and then choose what battery system to replace it with. Now that day have come as the old battery gave up a few weeks back. I have got a temporary replacement AGM battery whilst I make up my mind on the matter of what battery system to go for.

All pros points for a 48V system for many reasons that I might leave out as it probably have been discussed here multiple of times and it would be to kick in open door for sure. BUT there is one thought in my head that I need to have more opinions on to be able to make up my mind.

A very short backstory to understand my usecase a bit better:
I did design this solar system to be very efficient/cm2 even in very poor solar exposure. That gives that I today even can get some power of the array even from the moon at special conditions... let's not dive into that rabbit hole but I just like to explain that the array do produces energy at very low sun exposure.
I live in the middle/south-middle of Sweden and we averages 4.95 hours of sun/day. My array is build up by 3*110W Monocrystal PERC panels in series having an open circuit voltage of 23.2V per panel (69.6V@25C VOC and at lowest temperature this array would ever be exposed to 83.38V@-30C VOC). I have a Victron SmartSolar MPPT 100/20 (12-48V) solar charger. Having the array providing a higher voltage then the current 12V battery do not require much sun rays at all as I have measured a few watts produced already at around 30V coming off the array that at full exposure operates at around 68-69V. I did measure this using the Victron connect app tonight as the whole day have been overcast and it started to poor down just as the sun set clockwise. I do have graphs showing that the array did procure in tonight very poor conditions 24-ish watts averaging 52V off the array. As the voltage off the array dropped further it still produced just short of 10W at 46-ish volts as well as 5-ish watts just short of 45V... at a voltage off the array averaging 29-30V it still produced 1-2 watts. I know that we are not talking about a lot of power, but still it is power that is going into the 12V battery as is today...

If I was to go for a 48V battery system then I feel like I'm going to loose out of the low sun exposure power as the array will drop below 56V (48V) quite quick compared to a 14V (12V) or even 28V (24V) battery system. What do You think? This is what I need Your thought on.

I do not run a large inverter today but plan with the new battery system to increase the inverter to something in-between 1500W-3000W depending on battery system.... My daily need when it comes to energy consumption is about 1KWh/day. I'm calculating with quite a few different configurations, naturally providing different level of resilience to poor solar days. My largest configuration is providing me with at least 4 days of no sun before I have to alter my power usage patterns. This is another rabbit hole that we can dive into at another time as I really like to focus on the possible drawback in regard to the potential loss of the low sun exposure power with a 48V over a 12V or possible a 24V battery system. Adding a fourth 110W solar panel, in series with the other three, will unfortunately push me over the limit of 100V with the solar charger taking the VOC voltage of the whole array at lowest temperature into consideration.

So what are Your thoughts om the topic?
Thanks in advance for sharing them with me and others in this forum.
/Toby

 

[HRTKD]

Your voltage drop off will preclude the solar charge controller from charging. The usual charge start voltage difference has to be +5 volts. If your battery is low on state of charge that may help with the lower start up voltage of the PV. Once charging starts, the voltage differential is much less, so you'll still be able to charge. But less than about 54 volts from the PV isn't likely to do much. As the voltage is dropping off, so are the amps. So you may not be missing out on that much power production anyhow.

If you're that concerned with PV production, it sounds like you need another panel (or six) anyhow. An upgrade to a bigger solar charge controller is in order.

 

[mr2web]

Your voltage drop off will preclude the solar charge controller from charging. The usual charge start voltage difference has to be +5 volts. If your battery is low on state of charge that may help with the lower start up voltage of the PV. Once charging starts, the voltage differential is much less, so you'll still be able to charge. But less than about 54 volts from the PV isn't likely to do much. As the voltage is dropping off, so are the amps. So you may not be missing out on that much power production anyhow.

If you're that concerned with PV production, it sounds like you need another panel (or six) anyhow. An upgrade to a bigger solar charge controller is in order.

Thanks for your reply HRTKD.
With that +5 voltage differential to trigger the charger to start charging, I would carefully state that a 24V battery system seem less likely to waist low sun exposure power then a 48v battery system, wouldn't you agree? I know its not a lot of power that we are talking about. Having a wider gap, from the array normal operational voltage at a good sun exposure, to the battery voltage could be more conservative and deliver that little extra power to the battery then the very much thinner gap to a 48v battery system with my array setup, wouldn't you agree?
It is a very small system so 10W is in my usecase for sure to care for collecting rather then ignore it.
This is the final crossroad in my selection in-between 24v or 48v battery system, its a strategical decision as everything else I like to upgrade in the system will have to follow in its tracks when it comes to b2b and mains charging as well as the inverter selection. If I would live where good sun exposure would be more guaranteed then where I do live it would not matter to me but most days here are overcast to some extent and those days with clear blue sky are lovely but quite rare except for in June and July that is. Adding to the array is not really a good option as my RV' roof is quite packed as it is. Which system would you go for? 24v or 48v?

 

[HRTKD]

I agree that a 24 volt system may charge better. But is it worth it? I would have to see a cost breakdown to answer that.

I'm in the keep it 12 volt camp but for different reasons than PV charging. 12 volt is less complex, requiring fewer additional components (step down converters for example) and other components that are less complicated (12-12 DC charger instead of a 12-24 DC charger).

There's a gray line for moving up to a higher voltage for a battery system. The need for 3000 or more watts of inverter is the main reason to go 24 volt (or higher). I'm running my trailer on a 2400 watt inverter just fine.

 

[mr2web]

I agree that a 24 volt system may charge better. But is it worth it? I would have to see a cost breakdown to answer that.

I'm in the keep it 12 volt camp but for different reasons than PV charging. 12 volt is less complex, requiring fewer additional components (step down converters for example) and other components that are less complicated (12-12 DC charger instead of a 12-24 DC charger).

There's a gray line for moving up to a higher voltage for a battery system. The need for 3000 or more watts of inverter is the main reason to go 24 volt (or higher). I'm running my trailer on a 2400 watt inverter just fine.

Ok, attached is a summery of 12v, 24v and 48v battery system divided into three classes; Budget, Balance and Best. Sorry for the Swenglish at times but I did not intend for anyone else to look at this as it is very specific to my usecase and system already present in my RV. But it gives you an overview of cost, weight, capacity and so on... having the capacity to survive up to four days, without any major change in the usage pattern, is something I value quite high.

 

[PDJ]

You should use a charger with a step up and step down converter, I made one myself, I use a 12V system, but it can charge a 24V system as well.. it starts charging at 6V input (step up mode) and can handle up to 60V input (step down mode) (it works automaticly so it give the correct output for your my 12V system from 6V input up to 60V input)
I'm not sure what model solar charger do have a combined step up step down comverter, but technically it is possible what you want

 

[mr2web]

You should use a charger with a step up and step down converter, I made one myself, I use a 12V system, but it can charge a 24V system as well.. it starts charging at 6V input (step up mode) and can handle up to 60V input (step down mode) (it works automaticly so it give the correct output for your my 12V system from 6V input up to 60V input)
I'm not sure what model solar charger do have a combined step up step down comverter, but technically it is possible what you want

You are right and it is quite strange that no solar charger from the bigger manufactures have a buck/boost functionality. I use a Victron Smartsolar charger 100/200 (48V capable). I really like this charger and I wish Victron would add boost functionality to their chargers as that would make them even more to stand out on the market.

 

[PDJ]

what current do you need to charge? if it's less then 20A you can try this:

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This is the one I used, but I rebuild it a bit and add a DAC to control current and voltage, but for LifePo4 you don't have to as you can set max current and voltage
This is a buck/boost converter