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Author Topic: Choosing batteries and inverters  (Read 6444 times)
KenB
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« on: October 08, 2007, 07:38:21 AM »

List,

Many people are faced with several decisions when they come to obtain suitable batteries and inverters for their renewable power project.

Here's a few notes for guidance based on my own experience.  I was working to a fairly tight budget so the batteries and inverter were sourced second hand.  Whilst this will keep the costs down there are several points to bear in mind.

I am using a Lister generator with a 130V dc output to charge the batteries, but the same points apply if you are using a solar pV or wind turbine system - all be it at a lower battery pack voltage, often 24V or 48V.


The first question should be how much electricity storage do I need, and size the battery accordingly.

I use about 7 or 8 kWh per day and so decided on a battery that was big enough to provide at least a day's worth of storage.

My battery consists of 36 Hawker SBS40 gel batteries wired into 4 parallel strings of 9 batteries. These when new, have a nominal 38Ah capacity (at the 10hour rate), but I suspect that mine are about half of this capacity - as some of them are 15 years old.

My inverter uses a nominal 108V dc input. The voltage can vary from 93V (almost flat), to 110V normal, and right up to 130V when connected to the dc generator and at the end of their charge cycle.

The battery voltage is very much dictated by the inverter's requirements. The higher power inverters use at least 48V if not 96V. This high voltage is in order to keep the costs of handling high currents (thick cables, heavy duty switches etc) down to a minimum.

If you want to run typical household loads from an inverter you should look for at  least 3kW. Don't be tempted by the cheap 12V inverters.  Taking 3kW from a 12V source will need about 300 amps of current, which means thick welding cable and expensive high current switches.

I used thick galvanised steel strapping to connect the batteries into their strings.  These straps are about 32mm wide, 4mm thick and are perforated with 6mm holes every 50mm. They were bought at a local builders merchants, and are ideal as the 50mm spacing of the perforations fits my battery layout perfectly.  They were much cheaper than copper, and as the currents are low and the straps are thick, there is very little advantage in going for the additional expense of copper.

The inverter converts the nominal 110V dc from the battery pack up to a clean sinusoidal 50Hz 235 V ac supply.  When you turn on an appliance in the house, the power to supply that appliance is drawn as a high current from the battery pack.  An appliance using 13A of ac, will use approaching 30A of dc from the battery pack. For example, when my dishwasher is running (with its heater on) from the inverter, it might be drawing 27.5A from the batteries at a voltage of 115V dc. The 27.5A is split equally across the 4 battery strings, so only about 7A current draw from each battery.

In this way the 3kW load is shared evenly across all 36 batteries, so each sees a manageable 84W draw.

If you are looking to install a battery and inverter system, it is often be advisable to find a suitable inverter first. Depending on your budget, you might find a cheap computer UPS on Ebay.  These contain inverter and charger and often come with batteries.   If you want to supply parts of your house, you will need at least 3kW.

Having got the inverter, that dictates what battery voltage you need.

If you are buying batteries from a scrap merchant you should check them first. Gel batteries are best as they have considerably less maintenance than flooded cells. They are almost "fit and forget".

The batteries should be tested under load.  One suggestion is to make a rig of 4 old diesel glowplugs wired in parallel across some heavy jump leads and a digital voltmeter. This is about a 600 W load at 12V and will try to draw about 50A from the battery under test.

Put the test load across the terminals of the battery for just 20 seconds and see what the voltage drops down to. If it plummets to 7V or less the battery should be rejected. If it stays between 10V and 12V then the battery still has some good life left and you can pay the scrapman his 5 quid, knowing that you have got a good buy.

If you look for batteries between say 60 and 100Ah you will need fewer of them for your pack.  These batteries are heavy, with a 65Ah unit weighing about 23kg. You will need an estate car, trailer or van if you are collecting them from a scrap dealers.  My battery pack for example weighs nearly 470kg - way too much for the average car to carry safely.

Be wary of buying cheap batteries on ebay without doing a load test first.  It is becoming more difficult to send batteries as waste, and unscrupulous persons might choose to sell you their very scrap batteries rather than paying for it to be disposed of in the correct manner.

That said, there are considerable numbers of sealed lead acid batteries arriving at scrap yards, that have sat for 2 years in a back-up power unit sitting at their correct float vltage, and never seen arduous active service - i.e. as good as new.  The dot-com boom of 2000, meant that a lot of UPS systems were installed in offices and these are now ending up in skips- hardly used and fully serviceable.



Regarding battery charging, there are several methods.

1. You can use a dc generator and run it at the correct speed to supply approximately 14V to each battery.

2. Alternatively if you are running an ST alternator from an engine it can be  wired up for 115V ac output which you then rectify for battery charging. By adjusting the field voltage you can in turn control the output voltage to give you a sensible charging current into the batteries. Alternatively run the ST at 230V and use a 115V site transformer to make the nominal 115V, or a suitable transformer for whatever battery voltage you need.

3. If you are lucky enough to find a computer UPS, then these often feature "double conversion" mode. This means that the ac input is converted to dc to float the battery pack at the correct voltage (about 13.8V across each battery) and then inverted back up to clean sinusoidal 50Hz to power the load.  This means that if the ac input fails, the UPS keeps the load running without a glitch for as long as the batteries last.

There will always be a loss of efficiency when using batteries and inverter. Mine is very inefficient at low loads, improving to about 90% efficient at full load. 

However some home appliances don't seem happy with the raw ac power supplied by the ST alternator, so a sine wave  inverter is often the only way to proceed.  If you don't want to run the engine 24/7, then some battery storage will be needed.

It also allows you to add other power sources at a later date - such as a wind turbine or solar pV and connect them together at the battery  It is easier to combine infrequent power sources such as solar pV and wind in the dc domain, than trying to source separate inverters that will synchronise their ac outputs - i.e. grid tie such as the Soladin series

If you want a bigger battery, then forklift truck batteries are one possibility with a capacity of roughly 48V at 800Ah.  These are likely to be flooded cells, so will need to be topped up with distilled or de-ionised water at regular intervals.


regards,


Ken
« Last Edit: October 08, 2007, 08:53:50 AM by Ken Boak » Logged
snozzer
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« Reply #1 on: October 08, 2007, 08:09:29 AM »

Way tpo go Ken, what a great post, got me thinking big time about a PV project on my roof.

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frotter
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« Reply #2 on: October 08, 2007, 10:27:21 AM »

Mmmmmm... forklift batteries.... Smiley



Just make sure you get ones with bolt on terminals unless you're keen on lots of heavy duty soldering.

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« Reply #3 on: October 08, 2007, 09:40:02 PM »

snap Wink

       

wet cells every time for me Grin

cheers, paul
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http://lifeattheendoftheroad.wordpress.com/

'Off grid' since 1985,  Proven 2.5kW, Proven 6kW direct heating, SI6.OH, 800ah Rolls, 4.75kW PV ,4xTS45, Lister HR2 12kW, , Powerspout pelton, Stream Engine turgo, 60 x Navitron toobs and a 1500lt store. Outback VFX3048 and 950ah forklifts for backup,
jason
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« Reply #4 on: October 13, 2007, 03:04:29 PM »

Hi totaly agree like wet cells but you have got to use what turns up at the right price.
i hav AGM batteries an dthey seem to be ok. time will tell
 jason
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camillitech
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« Reply #5 on: October 13, 2007, 10:27:01 PM »

Hi totaly agree like wet cells but you have got to use what turns up at the right price.
i hav AGM batteries an dthey seem to be ok. time will tell
 jason

Hi totaly agree like wet cells but you have got to use what turns up at the right price.
i hav AGM batteries an dthey seem to be ok. time will tell
 jason

can't argue with that jason Wink  but if your buying new cos you don't have the national grid to fall back on and your wifes not very good with a multimeter then traction batteries every time Cool, AGM, spiral wound, sealed, carbon fibre, gel etc, etc are fine if your short of space, need to turn them upside down , eat your dinner off them, leave them unattended on a shelf for years, want to submerge them  or need something light, but if you need to charge and discharge rapidly and repeatably then fork lift truck batteries hit the spot Wink  ok you've gotta top em up but water's cheap Grin


regards, paul
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http://lifeattheendoftheroad.wordpress.com/

'Off grid' since 1985,  Proven 2.5kW, Proven 6kW direct heating, SI6.OH, 800ah Rolls, 4.75kW PV ,4xTS45, Lister HR2 12kW, , Powerspout pelton, Stream Engine turgo, 60 x Navitron toobs and a 1500lt store. Outback VFX3048 and 950ah forklifts for backup,
frank2
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« Reply #6 on: November 14, 2007, 09:21:36 PM »

hi
i,ve just bought one of those computer ups units, ( i,m only tinkering with a yanmar deisel and car alternator).
its an apc 620, from speedie computers b/ham, 15 quid and i,m quite impressed !
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