Which Inverter?

[JonB]

Hi, bit of advice needed (adam2?). Need an inverter to run off a deep cycle battery for fridge and boiler back up. Have a 75Ah deep cycle I use for a pump to put veg oil in the car. Now need an inverter to attach and plug in in case of power cuts. Longer term, line from our grid tied solar PV comes into the garage, so in a powercut ridden future can get this changed. Happy to get another battery if needed.
I would rather have 2 smaller inverters and another battery than start building a system. Would also give redundancy / backup.

Had look online, and don't like build quality of the ones I've seen so far, so recommendation welcome. Would also get overspecced if this will help.

Thanks

[adam2]

Any reputable make of pure sine wave inverter would be suitable.
Avoid "modified sine wave" which is a marketing term for square wave and is unlikely to be suited for either a fridge or a boiler.

Xantrax are the brand leader.

I would suggest at least a 1KW inverter in order to allow for the starting current of the fridge.
I doubt that the steady running load of the boiler and the fridge will exceed 300 watts in total. It would however be advisable to measure the load of both appliances before deciding on the inverter size.

I doubt that a single 75 A/H battery will suffice unless for very short term use under favourable conditions.

Ideally you need to determine the running load in order to size the inverter. Select an inverter of several times the running load in order to allow for starting currents.

Then measure the typical consumption in KWH over say 12 hours in order to size the battery bank.
preferably measure in both hot and cold weather.
I would select the battery to supply the load for 12 hours, if fully discharged. REGULAR deep discharge will seriously reduce the life of the battery, but I would not worry too much about this if planning for 12 hours without power.
A 12 hour power cut will hopefully be a rare event, and a more typical cut of a few hours will discharge the battery much less deeply.

A bigger battery will of course protect against longer power cuts, but in general, for loads like this a generator is a better bet for more than 24 hours.

[JonB]

Any reputable make of pure sine wave inverter would be suitable.
Avoid "modified sine wave" which is a marketing term for square wave and is unlikely to be suited for either a fridge or a boiler.

Xantrax are the brand leader.

I would suggest at least a 1KW inverter in order to allow for the starting current of the fridge.
I doubt that the steady running load of the boiler and the fridge will exceed 300 watts in total. It would however be advisable to measure the load of both appliances before deciding on the inverter size.

I doubt that a single 75 A/H battery will suffice unless for very short term use under favourable conditions.

Ideally you need to determine the running load in order to size the inverter. Select an inverter of several times the running load in order to allow for starting currents.

Then measure the typical consumption in KWH over say 12 hours in order to size the battery bank.
preferably measure in both hot and cold weather.
I would select the battery to supply the load for 12 hours, if fully discharged. REGULAR deep discharge will seriously reduce the life of the battery, but I would not worry too much about this if planning for 12 hours without power.
A 12 hour power cut will hopefully be a rare event, and a more typical cut of a few hours will discharge the battery much less deeply.

A bigger battery will of course protect against longer power cuts, but in general, for loads like this a generator is a better bet for more than 24 hours.

Thanks.
Total house use with both less than 300 watts with just about everything else off. So I'll assume 300 for safety.
Will oversize inverter as recommended. Something over 1 kW, then.
So if I've got it right, 75 Amp hour battery x 12 volts = 900 watt hours, so the battery will last 3 hours at 300 watts. Roughly.
So for a 12v supply for 12 hours at 300 watts needs 300 Amp hours battery(ies). Not allowing for conversion losses. 25% for that?

Largest batteries I've seen are 100 Amp hours locally. I plan to have 3 of these, and swap the inverter over as exhausted. Or is that a mistake? I don't fancy wiring always live stuff together/

[woodburner]

There are only two types of open lead acid batteries. The standard ones, and traction batteries. The "deep cycle" batteries are a marketing exercise and will not give the benefit the extra cost appears to indicate.

See http://www.sterling-power.com/support-faq-2.htm

Watch the term leisure / deep cycle as it simply does not exist. The standard, so called, leisure batteries, are simply starter batteries with extra support for the active lead material. This may increase the life by 5 – 10 %, but does not turn a starter battery into a deep cycle battery. True traction (deep cycle) are not available at a sensible price and are uneconomical to use for standard leisure use. However, if you plan to live onboard or travel the world then do look at 6 V or 2 V traction and build your battery bank up from those batteries, but expect to pay about 3 – 6 times the price of so called standard leisure batteries. On a daily use cycle, the standard so called leisure battery (which is a starter battery) will last you as little as 6 – 8 months whereas traction would last 15 years

[adam2]

If a lead acid battery is discharged relatively quickly then the capacity is much less than than the nominal capacity.

If you have/intend to aquire 3 batteries each of 100 A/H, then I would permanently connect them in parallel so as to give a 12 volt battery of 300 A/H.

If the load is 300 watts, then that will be about 30 amps at 12 volts.
A 300 A/H battery should supply 30 amps for 9 or 10 hours.
A single 100 A/H battery would probably only supply 30 amps for about 2 hours, at the most.

[JonB]

If a lead acid battery is discharged relatively quickly then the capacity is much less than than the nominal capacity.

If you have/intend to aquire 3 batteries each of 100 A/H, then I would permanently connect them in parallel so as to give a 12 volt battery of 300 A/H.

If the load is 300 watts, then that will be about 30 amps at 12 volts.
A 300 A/H battery should supply 30 amps for 9 or 10 hours.
A single 100 A/H battery would probably only supply 30 amps for about 2 hours, at the most.

Thanks again. More questions: So, is there a off-grid for dummies I can use to safely put this together? Are RS a good place to get kit, or something better on line. I want to do this right. I have the space and know where I can put the batteries.

[adam2]

RS are a reputable supplier, but are sometimes rather expensive.

I would be be pleased to give more detailed advice either by PM or in this thread.

There is nothing complex in what you require, but since mistakes can be fatal, it would be well to be cautious and take advice whenever in doubt.

[SleeperService]

To reinforce adam2's warning the DC side can be dangerous not just the AC side. People get killed by fork lift batteries quite often enough

[woodburner]

So, is there a off-grid for dummies I can use to safely put this together? Are RS a good place to get kit, or something better on line. I want to do this right. I have the space and know where I can put the batteries.

sterling-power.com seems a good place to start. At least the guy tells the truth.

[kenneal - lagger]

Hi! Jon

I've recently bought a 3kW Sterling Pro Combi inverter from www.furneauxriddall.com and it is working well. I was very impressed with the service from Furneaux Riddall and Sterling as the original inverter was delivered with a dent in the chassis which affected the running of the cooling fan. There was no dent in the box so the unit must have been dropped at the manufacturing plant. Furneaux Riddall arranged for a new one to be delivered almost by return, no questions asked and delivered direct to us to save time.

They are a boat chandlers so have all the 12V switching and fuse kit that you will need so are a good company to work with.

Our Powermaster inverter has developed a fault in the charge controller so I'll see if it can be fixed. The one thing I don't like about the Sterling inverter is that it doesn't have a low battery cut out like the Powermaster so buy a volt meter to go with it. The Sterling unit seems to charge the batteries better though.

When sizing your batteries you should only allow them to discharge to 50% max, to about 11.6V or 11.4V minimum, so if they're 100Ahr batteries only use 50Ahrs. In effect you need twice the number of batteries for any given load. At that level of discharge they should last 5 years instead of one or two, if you're lucky. It's well worth the extra expenditure initially.

If you buy good quality sealed deep cycle batteries you'll have to order them as they are made to order and Furneaux Riddal can supply these as well. You will also need a charge controller for the PV cells as your grid tie inverter/controller won't do. You'll probably need an MPPT controller so that you can take the series wired panels, at a relatively high voltage, and put them into your 12V batteries. You may still have to rewire the PV panels to get the voltage down to about 100V to 120V.

Good luck.

[adam2]

To reinforce adam2's warning the DC side can be dangerous not just the AC side. People get killed by fork lift batteries quite often enough

Fork lift batteries are often of a relatively high voltage, high enough to be dangerous, and great care should be taken when working on fork lift or other electric vehicle batteries.

A 12 volt battery such as is proposed by the O/P can not give a dangerous electric shock under any reasonably forseeable circumstances.
Care should still be taken even with 12 volts, the risks are

1) fire from overloaded wiring or short circuits. Use correctly sized fuses to avoid thi.

2) fire and explosion from errors in connecting batteries together. Take great care and plan the work very carefully indeed.

3) explosion from the hydrogen gas evolved by batteries on charge.
Either use sealed batteries, or provide ample ventilation.
Use a charge controller to avoid overcharging.
Keep flames and sparks away from batteries.
Avoid switches, fuses, relays and the like in a battery enclosure (unless non sparking)

[Catweazle]

If you have/intend to aquire 3 batteries each of 100 A/H, then I would permanently connect them in parallel so as to give a 12 volt battery of 300 A/H.

Remember to fuse them individually, otherwise if one develops a fault the others can dangerously discharge through it.

[JonB]

Thanks to all . Lots of help and lots to think about. As with previous preps, the decision is the easy bit, now the work begins. I have friends with a canal boat who are good on 12V, and they will help. Ken, I think using the "marine type hook up" is the best solution. I like the units that run on mains then autoswitch when cut. I think with a small mod I can put the circuit with the fridge and boiler on this so it would be automatically switched over.

The other bits I need to read up on. Is there a guide out there from someone who has done this? i.e. stay with the grid tie then battery back up?

If anyone has done this, would appreciate seeing your system.
Adam2, if I can I will PM you when I have a plan for which I would appreciate your expert critism.

Thanks again

Jon

Exit left: Skulks off to plan getting this past the missus, not mentioning threads about exploding battery risks...

[adam2]

If you require to use the grid normally, but want an automatic backup supply, then you might do better to use an "off the shelf" UPS.

These are primarily intended for backup power to computers and the like, they typicly give a run time of from 5 to 15 minutes from the small internal batteries.

UPS units are often discarded or sold very cheaply when the internal batteries fail.

It is a simple matter to discard the failed batteries and fit much larger external batteries so as to give a much longer run time.
There are however a number of drawbacks to use of a UPS in this way.

Firstly you will be paying forever for the small but real losses in the UPS.

Secondly, the inverter is probably only rated for short term use, limited by the small internal battery. Prolonged use at full power will probably kill the inverter due to overheating.

Thirdly the battery terminals MAY NOT BE RELIABLY ISOLATED FROM THE MAINS, these must be regarded as mains connections and protected against being touched.

Fourthly, computer type UPS units are not good at motor starting and need to be very oversized if feeding motors.
A UPS capacity of TEN TIMES the largest motor load is advisable.

For a fridge and a central heating system, I would suggest a UPS of about 2,500VA or 3,500VA.
These appear on ebay for less than £100, with missing or dead batteries.
The external battery bank must of course be of the same voltage as the internal battery that is removed. This is often 24 volts, but double check.

The external battery and the leads therefrom should be protected by a suitable fuse, preferably one designed for mains use.

I would suggest an APC branded UPS, these are very popular and often turn up used.

[JonB]

Thanks. Will into that option,too.