optimal system voltage

[gothicsurf]

I am just beginning a plunge into building out the shell of my truck as a robust boondocking/camping or if need be survival shelter and am stoked about figuring out the electrical setup. So far I have 2 100W compact Renogy solar panels, a 20A 12/24V Rover Elite MPPT and 2 35Ah SLA batteries. I plan on placing the batteries behind the bench of the cab. Because there are two batteries and the controller is 12/24V capable, I got to wondering the value of running them in series for 24V. Perhaps this is more efficient for the MPPT and inverter?

First I do plan to charge them from the alternator in parallel when the car is running using a relay disconnect as shown in this video actuated by the 12V ACC lead of the ignition. Would it make sense to then connect the MPPT and inverter to these in series, and any other 12V appliances in parallel? Maybe this would provide a more stable voltage level or lessen the effects of a deep drain? Or would this thought really only save on a bit of copper in being able to run a couple connections in higher gauge?

 

[Weight]

On larger systems 24 volts can lend more efficiency. You can have 24 volt solar charge controller. Would require enough panels to power it. You can have a 24 volt inverter. Under 2000W it is questionable if more efficient. Then there is that 12 volt alternator. It takes an expensive device to get 12 volts up to the 24 volts needed to charge a 24 volt battery. Then the other stuff that needs 12 volts would need an inexpensive device to lower 24 to 12. That is why I staid with the vehicle's native 12 volts. I only use the inverter a few minutes a day.

 

[highdesertranger]

the important question is what to you want to power? that should always be the first step. highdesertranger

 

[barleyguy]

A 200 watt system is not large enough to go to 24 volts. You'd normally want to go to 24 volts when you're getting over 50 amps or so, because at that point you're getting to the upper edge of what connectors and wiring are rated for. 24 volts will be about 1% more efficient for the same quality inverter, but you can get that same 1% by going up in quality, or by just turning off the inverter one percent of the day. Inverters generally won't do multiple voltages, so you need to decide your system voltage before buying the inverter.

Another advantage to 12 volt is that you have the future option of going to a pair of 6 volt batteries, which is often the best option for SLA/AGM. Also, if you ever decide to go to lithium, many lithium batteries can't be run in series (the expensive top name ones can, but cheaper ones can't), so you're limited to 12 volts.

In short, IMO you should stick with 12 volts unless you're building a large system with expensive components and batteries.

 

[kygreg]

If you want to charge with you vehicle alternator, I would just stick to 12v. I'll have 800 watts of solar panels and a pretty big battery, for me 24v has enough advantages to go with, but I'm not trying to change with my alternator.

 

[Spaceman Spiff]

I agree with HDR.
In any engineering problem you start with a needs analysis, in this case 'what are you powering'?
That will determine your battery voltage.(unless you want to spend the money on voltage modifiers).
Then you continue upstream, deciding the best components for what you want your system to do.

 

[gothicsurf]

ohp forgot that minor detail .. far as the main load, it would be a 20L compressor fridge which draws 3.5A (maybe 20% of the time depending on the ambient) then a couple 120mm fans while cooking, some kind of led lighting, and several misc smaller things like charging a phone, headlight, etc.

I guess the bulk of my inquiry is the possibility of forgoing use of converters/transformers if two batteries are used and connected in parallel to the alternator and series with the mppt and inverter.

The other significant draw is possiblly the charging of an ebike battery. This is easily accomplished with a standard charger through the inverter, but would like to know any reason, in the area of efficiency, to entirely switch the mppt from the house battery to charging the lithium ebike battery. Though guessing it would be too much trouble resetting the output of an mppt capable of higher voltages.

funny but this is a screen capture of a youtube solar bike touring video where he shows his solar charging setup. It looks like two devices are necessary, unless this is a 12/24V mppt with a hobby charger used to step it up to 36-52V. Maybe a higher voltage mppt would get the job done?

 

[highdesertranger]

1. figure out your daily usage
2. size you battery bank according to #1
3. size you solar according to #2

highdesertranger

 

[Trebor English]

I guess the bulk of my inquiry is the possibility of forgoing use of converters/transformers if two batteries are used and connected in parallel to the alternator and series with the mppt and inverter.

Pick one, series or parallel.  You can set up fancy switching with off between series and parallel but the switch contact resistance and extra wire resistance would eat up any saving you would get by using 24 volts vs 12 volt.  Don't forget, you really want to disconnect the solar input to the MPPT controller before it gets disconnected from the battery while switching.  You could do it with relays so that the series relays must be off before the parallel relays can turn on and the parallel relays must be off before the series relays can turn on.

parallel: 
A+  --  B+
A-  --  B-
series:
A+  --  B-

both at the same time:
A+  --  B+
A-  --  B-
A+  --  B- 
and remember that A+ is also connected to B+ so all the plusses and all the minuses are all tied for one big rapid discharge.

 

[gothicsurf]

So I haven't looked at the wiring thoroughly, but this idea of a series/parallel hybrid 12/24V battery hookup generally isn't feasible?

 

[Weight]

no it is not

 

[B and C]

Stick with a 12V battery bank. Everything will be much easier and simpler.