Should I go off-grid?

HertsLad

A named storm was due but hasn't yet arrived. Strong winds are rare in Hertfordshire and might just provide useful power but it seems too unpredictable. Nevertheless, my most recent thinking was to play around building a wind tower up to 5m high with home made blades of 2m or 2.4m. This would drive either a home made generator or a car alternator with the regulator removed.

Ectophile

HertsLad said:

A named storm was due but hasn't yet arrived. Strong winds are rare in Hertfordshire and might just provide useful power but it seems too unpredictable. Nevertheless, my most recent thinking was to play around building a wind tower up to 5m high with home made blades of 2m or 2.4m. This would drive either a home made generator or a car alternator with the regulator removed.

I hope you're going to apply for planning permission if it's going to be more than a temporary thing.

HertsLad

Ectophile said:

HertsLad said:

A named storm was due but hasn't yet arrived. Strong winds are rare in Hertfordshire and might just provide useful power but it seems too unpredictable. Nevertheless, my most recent thinking was to play around building a wind tower up to 5m high with home made blades of 2m or 2.4m. This would drive either a home made generator or a car alternator with the regulator removed.

I hope you're going to apply for planning permission if it's going to be more than a temporary thing.

Of course not! I will claim it's only a temporary R&D set up, even if I leave there for months or years. My expectation, though, is that it would only produce any useful output very few times a year, when the forecast is for strong winds. So, even if I put in a considerable effort to construct it, I doubt if I will think it's worthwhile, long term.

HertsLad

Does anyone here have an opinion on safe discharge levels for 12v lead acid batteries? I hope it's only or primarily the voltage without a load which counts, I understand 12,2v is probably the safe lower limit for about 50% discharge,
Example: a battery is at 12.5v with no load. A load is then applied and pulls the voltage down to 11.8v. Is that 11.8v relevant, or in any way a concern, if the voltage recovers to 12,4v a few minutes after the load is removed?

QrizB

HertsLad said:

Does anyone here have an opinion on safe discharge levels for 12v lead acid batteries?

It depends on the battery design and chemistry.

Ideally you'd be monitoring Ah in and out, and using that to limit discharge rather than relying on voltage.

If you have to use voltage, it's best to use open-circuit (no load) voltage.

In your example, the 11.8V isn't important as a measure of depth-of-discharge, but might tell you something useful about the battery condition if you also know the applied load (in Amps) that caused the 0.7V drop.

Edit to add: it's not perfect but the RV wiki gives a reasonable overview of 12 volt life:

http://rvwiki.mousetrap.net/doku.php?id=electrical:12v:intro

(It isn't a user-editable wiki, it's maintained by a US-based van-living retiree.)

HertsLad

QrizB said:

HertsLad said:

Does anyone here have an opinion on safe discharge levels for 12v lead acid batteries?

Ideally you'd be monitoring Ah in and out, and using that to limit discharge rather than relying on voltage.

...the RV wiki gives a reasonable overview of 12 volt life:

Many thanks. I will look at the RV wiki link in a moment.

I tried monitoring Ah in and out using a Victron battery monitor which they claimed did just that. But I ended up not trusting it and considering it a case of data overload.

Qyburn

It's a right bag of worms trying to assess state of charge based on voltage. For any one state of charge the voltage will depend on current going in or out, and is even affected by what the battery was doing in the past.  

I can't find it just now but Squadron Batteries used to have charts for each type showing expected life in cycles vs depth of discharge. From memory there was no completely safe level, it just varied the life. Again from memory something like 500 cycles if discharged to 50% for their standard flooded lead acid leisure range.

HertsLad

Yesterday, I re-arranged my solar PV system to see if it works even better than it did before.

The previous system comprised 6 solar panels (equivalent) fed into a single battery bank, with all devices and appliances needing power, drawing it either 24v direct or through inverters. The battery bank was flooded lead acid, supported by a couple of small lithium batteries.

I decided to split it into 3 sub systems. These are fridge, freezer and other.

The fridge now runs on 24v from two lead acid batteries (70ah each) in series. Power comes from 400w of solar panels, half horizontal and half south facing. The controller is a Victron MPPT.

The freezer now runs on 230vac from a pure sine 1500w inverter which is permanently running. The inverter is supplied with 24v from two lithium batteries (35ah each). Power comes from 600w of solar panels which are currently facing north west. The controller is a second Victron MPPT. The fridge and freezer systems are quite similar. I thought it would be interesting to see for myself how lead acid and lithium batteries compare. I believe the battery capacities are similar because I could use most of 35ah from the lithiums but only 50% of 70ah from the lead acid.

The 'other' sub system is about 12 flooded lead acid batteries in a 24v configuration, feeding a 4000w pure sine inverter which is turned off 90% of the time.I am using it right now to type this message, on my desktop computer. Other uses are microwave oven, air fryer and power tools but only when the sun is shining or I expect it to do so soon.

Data after the first night was favourable. Neither the lead acid or lithium battery voltages fell much at all.

HertsLad

I have now re-arranged my solar PV system, yet again. With a system pulled together myself, it's not difficult to do. If I'd paid vastly more to have contractors fix panels in a (sub-optimal) permanent position on my house roof, I would have none of the flexibility. Neither would it be easy, either, to change the battery types or the way they are wired.

I have now removed my lithium batteries from their support role in holding up the voltage of my lead acid batteries, as promoted by a few people on youtube. It seems more trouble than it's worth. I'm sure I can find a better use for the lithium batteries.

Another change is that most of my solar panels now feed into two inverters. I guess if both inverters need to draw power, they need to figure out how the solar energy is shared. But, a lot of the time, only one inverter is turned on, so it receives 100% of the solar power.

mickyduck55

Interesting, will you not need more solar for the winter months?