Batteries chargers & inverters

[Justme]

Hi,

 

First I would like to saw that I have been reading this forum for 2 days solid looking at all the threads about this topic. I have learnt so much more here than I have on any other sites ( like off grid type ones) that purports to understand batteries, chargers & inverters.

 

My initial plan was to have a large battery bank with an inverter & a charger powered via a diesel genny. The numbers are

 

240v daily consumption 1.8kw/h via inverter (or 12v x 166amps)

plus

upto 12kw/h via genny over 4 hour period

 

I originaly planned to do this with

a large battery bank

1000w inverter

50amp charger

genny

 

 

After reading this site I now know that 4 hours of the genny powering a 50 amp charger will not produce 200amps into the battery. A 50 amp charger only produces 50amps for a small amount of the charge time.

I also know that for each amp I use out of the battery I need to input 1.5 amps

 

 

So my questions now are

 

1, If I need a 166-200amp daily supply at 12v & I want to keep the genny to 4 hours or less daily (no shore power or engine charge avaliable) realisticaly what size charger do I require?

 

2, As my battery bank is 12v x 1550amp (C20) (made up of 6 x 2v 1550 amp cells) would I be better using the batteries for 2 or 3 days then charging for longer? If so I would need to add another 80amps per day to the 12v comsumption as one item would need to be added to the 12 v supply (so 240amps daily).

 

3, Inverter size needed is only less than 500w but I know that I need much more to start some items (was looking at 1200w but have been offered a good deal on 3500w/7000w peak). As modern inverters have about the same base load across a large power out size is the old advice of have two inverters still valid?

 

4, Is it possible to use more than one charger into one battery bank? Reason I ask is that I can get an inverter with a 50 amp charger for a good price but feel that I will need more charging capacity than that. Similar to having wind, solar & a charger set up maybe?

 

I have a 5.5 kva genny so dont think a big charger will be a problem even though it will also be powering other devices.

 

I know that I need to fuse the 12v lines (mega / mini fuse)

 

Any other advice?

 

 

Justme

(Richard)

Edited May 24, 2008 by Justme

[Gibbo]

So my questions now are

 

1, If I need a 166-200amp daily supply at 12v & I want to keep the genny to 4 hours or less daily (no shore power or engine charge avaliable) realisticaly what size charger do I require?

 

This is far from a final answer because you've touched on a subject that is hugely complicated and could easily fill a set of encyclopedias.

 

You've got an immediate problem here. That of generator run time. It's going to mean you having to rethink your plans and coming up with another solution.

 

The problem is that, no matter how big your charger and no matter how big your generator, there is a minimum time that it takes to recharge lead acid batteries. And, unfortunately, that time is MUCH longer than 4 hours. It makes little difference how big the charger is. Well that's not entirely true, it does make a difference, but not as much as one would think.

 

To give you an idea, from 50% state of charge to 100% is NEVER going to be achieved properly in less than about 8 to 10 hours and preferably much longer. It doesn't matter if you throw a million amp charger at it, the batteries still need a long time at acceptance voltage to get them charged.

 

The deeper the batteries are discharged the more charge you can get into them initially. So a 1000Ahr bank could easily accept (say) 200 to 400 Amps charge current when flat, but as the charge state rises, the amount they will accept reduces. So the time to get the charge in increases hugely as they come up to charge.

 

The result is that it takes quite a short time to charge from 50% to 60%, it could be done in about 20 minutes or so. But to get from 90% to 100% will take hours irrespective of how big the charger is.

 

So now you need to rethink all your plans

 

The long and short of it is that you've got no chance of recharging lead acid batteries in 4 hours.

 

Gibbo

Edited May 24, 2008 by Gibbo

[Justme]

So my alt plan of 2 or 3 days use then a longer charge is a better one?

 

Is using the battery from say 50-80% repeatedly with occasional extra long charges to 100% viable?

 

Justme

[Gibbo]

So my alt plan of 2 or 3 days use then a longer charge is a better one?

 

Almost certainly.

 

Is using the battery from say 50-80% repeatedly with occasional extra long charges to 100% viable?

 

It's the way it's normally done when relying on an engine of some sort for charging. It's called "short charging". You use the generator at the part of the discharge cycle where you can get some serious charge into the batteries. Seeing a generator work had for it's money is quite rewarding. Seeing it sit there running for hours and hours to get from 90% to 100% and only getting a few amps into a battery bank is quite soul destroying.

 

There are (as always) a few pitfalls.

 

Firstly it isn't as good for the batteries as they tend to sulphate up more. This is really the biggest problem. It means you have to run an equalisation (deliberate overvoltage charge) at much more frequenct intervals.

 

When you do run a full charge you need to do it for a *long* time. Something like 18 hours or so at full charge voltage.

 

That is a bit of a nuisance with a generator.

 

It's a bit of an art and no hard and fast rules can be laid down as each installation is so different. Your best bet would be to consider this option and use a hydrometer to keep an eye on what's happening until you've come up with a workable system.

 

A consistently low SG reading is seriously going to shorten the life of the batteries.

 

Gibbo

[smileypete]

Hi,

 

I've thought about this a bit, the best I can come up with is to have two (!) banks; charge both daily but use one. Then once a week switch banks so the other gets used, by which time it should be 100% charged.

 

More batteries, but the second bank gives some sort of backup, especially if each bank uses single 2V cells.

 

A lesser way might be to use a secondary bank to complete charging on the first one once a week.

 

Some further reading on battery charging here: http://www.localenergy.org/pdfs/Document%2...0Efficiency.pdf

 

cheers,

Pete.

[Gibbo]

Hi,

 

I've thought about this a bit, the best I can come up with is to have two (!) banks; charge both daily but use one. Then once a week switch banks so the other gets used, by which time it should be 100% charged.

 

That's a really good idea. I think I shall amend my webpage about *not* splitting battery banks to include that as one possible exception ie where charging from an engine is being totally relied upon it's a method of ensuring that the batteries do actually get 100% charged.

 

Nice one!

 

There is however one proviso. Two independant chargers are needed or one charger with two *proper* independant outputs which are extremely rare otherwise the bank being used will hold the charge voltage down and prevent the other one reaching 100% anywhere near as quickly as it otherwise would.

 

Still a damned fine idea though.

 

Gibbo

[smileypete]

There is however one proviso. Two independant chargers are needed or one charger with two *proper* independant outputs which are extremely rare otherwise the bank being used will hold the charge voltage down and prevent the other one reaching 100% anywhere near as quickly as it otherwise would.

 

True. The charger for the 'out of use' bank could be smaller, as it will be doing an absorption charge mostly.

 

 

Another way is to constantly charge the 'out of use' bank from the 'in use' one using a small inverter and charger (or a 'boost converter')

 

Less efficient but the 'out of use' bank will reach 100% charge much quicker.

 

Not quite as bad as it first sounds, as the 'in use' bank starts at 100% and so will have more charge to spare.

 

cheers,

Pete.

Edited May 25, 2008 by smileypete

[Gibbo]

Another way is to constantly charge the 'out of use' bank from the 'in use' one using a small inverter and charger.

 

Yes, you mentioned this some time back.

 

Having thought about it properly I do believe that the advantage of actually charging the second bank up to 100% instead of having it hovering round 70 to 80% does indeed outweight the disadvanatge of the wasted power in the conversion process.

 

Use a small charger in relation to the battery bank size (say 10Amps for a 500Ahrs bank) then the charger would be working quite hard most of the time and therefore at its most efficient. The inverter then being chosen to be running at (say) 85% rating so it too is at its most efficient. The losses form this would actually be quite small. Especially if an automatic means was devised to switch off the inverter for (say) 24 hours once the charger was in float.

 

Or perhaps.........

 

One of those switch mode 12 to 12 volt converters with an output that can be above and below the input. Tweak the output to (say) 14.4 volts and arrange it so it only switches on when the other bank is being charged. Fewer conversion processes, narrow voltage range conversion and therefore much more efficient. Also probably much cheaper.

 

It's an idea worth further consideration.

 

Gibbo

[Keeping Up]

Good one Pete, the first bit of truly original thinking that I've seen on this topic for quite a while.

[smileypete]

Thanks.

 

I think it's worth considering if a battery bank is getting a little tired.

 

The usual option is to scrap it even though there's some useful life, then worry about keeping the new bank charged properly.

 

By putting the new bank alongside the old one and using them alternately it can give the best of both worlds.

 

Some more use can be had from the old bank, and both will get charged properly on a regular basis.

 

cheers,

Pete.

[Justme]

4, Is it possible to use more than one charger into one battery bank? Reason I ask is that I can get an inverter with a 50 amp charger for a good price but feel that I will need more charging capacity than that. Similar to having wind, solar & a charger set up maybe?

 

Justme

(Richard)

 

Any one?

 

Tried a seach for multipul chargers but no results

 

Justme

Edited May 25, 2008 by Justme

[Gibbo]

Any one?

 

Tried a seach for multipul chargers but no results

 

Justme

 

There is no immediate problem with 2 chargers connected together such as damage to each other. That's an old wives' tale.

 

But some of the modern ones do clever tricks which means that often one of them shuts down when two of them are connected to one battery bank. It all depends on the chargers.

 

Gibbo