The perfect battery/charger system

[magnetman]

I've always had this idea in my head and looking at some other suggestions on different electrics threads I wonder if it might work:

 

4 domestic batteries say 110Ah each.

 

split into 2 separate banks.

One bank is used for providing power to lights appliances inverter etc.

The other is used for putting current into by charging. when the charged bank is ready they would be switched over somehow.

That way a full charge could be given to a battery bank which is not providing any power to appliances and therefore could accept a higher charge voltage which apparently would cause them to charge quicker and last longer. It seems a bit counterproductive to be charging batteries that are being drained at the same time...

 

 

Would this work? How to desgin the changeover system is my main concern.

[Sir Nibble]

It seems a bit counterproductive to be charging batteries that are being drained at the same time...

You don't. If the batteries are getting 20A of charge and there is 10A of load switched on then the alternator will be making 30A. If you leave all of your batteries connected then the voltage for a given discharge will drop in a flatter curve due to the higher capacity.

Get ready for the explanatory maths!

[magnetman]

You don't. If the batteries are getting 20A of charge and there is 10A of load switched on then the alternator will be making 30A. If you leave all of your batteries connected then the voltage for a given discharge will drop in a flatter curve due to the higher capacity.

Get ready for the explanatory maths!

 

What I am thinking is:

 

1. Higher voltage would charge quicker apparently (milkfloat comment in another thread) but would blow lights and inverter ?

2. Smaller bank with no load would take shorter time to completely charge with a 4 cycle charger ?

 

I have got maths ability I can even use all the scales on a loglog slide rule (including LL001-003) but I don't know much about battery technology.

Bring it on!!

[chris w]

We have discussed the Peukert effect ad nauseum on here so I won't repeat all the maths - although it is tempting! Suffice it to say that you would lose around 20% effective capacity if you split the batteries into two banks rather than having one large bank.

 

ie: one large bank has a greater effective capacity than two smaller banks whose total capacity is nominally the same as the larger bank.

 

Snibble is quite correct about the alternator/charger output. The charger output will simply increase (up to its maximum design current) to offset any loads being used whilst charging.

 

Chris

[charles123]

This what I suggested some time back except I would have two alternators each separately charging a separate battery bank, each bank either powering everything and a changeover switch or separate banks one ballanced on demand serving say lighting and fridge with the other separate bank serving othres such as sockets etc

 

Need the electricity experts to say if its possible

 

Charles

[chris w]

See the reply I gave some moments ago

 

Chris

[magnetman]

There's a link from wikipedia to an explanation of p's phenomenon, and it goes quite deep. with a link to more. Modification of the original equation looks interesting, is this allowed? I've decided to put away my Sunn Hemmi slide rule and sit back and enjoy the fact batteries aren't very expensive for if they were I would be IDS.

[Lophat Euro]

From the impenetrable exchange I have just read, it seems you guys know about battereis. Can you advise me?

For the last six years I have left my battery charger switched on for the

off season

: six months. As the boat was moored 800 miles from home, I did not visit during that period. Batteries seemed Ok but I thought I ought to replace them. My new mooring does not offer free electricity, so I have not kept the charger on. Nonetheless, after four months, the batteries were fully charged when I visited last week ( new mooring is only 250 miles away). I was thinking about a solar panel to keep the charge - but on seeing the batteries fully charged, I thought, why bother. Should I bother?

Lophat

[Gibbo]

From the impenetrable exchange I have just read, it seems you guys know about battereis. Can you advise me?

For the last six years I have left my battery charger switched on for the : six months. As the boat was moored 800 miles from home, I did not visit during that period. Batteries seemed Ok but I thought I ought to replace them. My new mooring does not offer free electricity, so I have not kept the charger on. Nonetheless, after four months, the batteries were fully charged when I visited last week ( new mooring is only 250 miles away). I was thinking about a solar panel to keep the charge - but on seeing the batteries fully charged, I thought, why bother. Should I bother?

Lophat

 

A few points.........

 

How do you know they were fully charged? As in, what did you measure to come to this conclusion?

 

According to conventional wisdom, a lead acid battery will self dicharge with the rate of discharge varying between different types. Wet cells being the worst often quoted at about 1% to 2% per week. AGMs being the best at about 0.5 to 1% per month. However it varies greatly between different makes, models etc. Assume it's somewhere between 0.1% and 10% per week and you'll be about right! It's the self discharge that wrecks the batteries. If they didn't self discharge they could safely sit there for years. Of course this is assuming there are *no* loads whatsoever on the batteries. So really it comes down to what loads are left on, how quickly *your* particular batteries self discharge and how often you can get there to give them a bit of a charge.

 

I ran some tests on this a few years ago. One battery was still at 90% charge state after 3 years standing and in good condition. Of cours this is impossible. But it happened.

 

Gibbo

 

There's a link from wikipedia to an explanation of p's phenomenon, and it goes quite deep. with a link to more.

 

Can you guess who did the maths for the "deep" one?

 

 

Modification of the original equation looks interesting, is this allowed?

 

Yes because it doesn't change the equation. It merely modifies it to allow it to operate correctly given the way batteries are now specified eg 100Ahrs @ 20 hours. They didn't used to be specified like that. The original equation only works on batteries specified in the old way. That lot too is explained on that link.

 

Gibbo

[Lophat Euro]

Gibbo' post . How do you know they were fully charged? As in, what did you measure to come to this conclusion?

 

Many thanks Gibbo for taking the time to deal with this. For me “technical” has the same meaning as juju. I perform the actions required and expect the great oompah to shower me with bounty. If told to light joss sticks and chant at the batteries, I would see it as no different to my assiduous watering (distilled, of course – but happy to use 4711, if the manual demands it) of the cells. I use Johnson’s baby Vaseline on the terminals and they would pass muster at Horse Guards. I will assume that as I water the batteries, they are wet cells. I bought them at the tractor spares depot, taking the cheapest I could find ( batteries are more expensive in France).

I know – sorry - I believe the batteries were fully charged, because when I switched on the battery management system, it ran through the checks and the magic eye turned green within four minutes. I was there for three days and had the same result each day.

As to what loads are left: I switch off the battery isolator switch; water pumps are switched off; and the phone is switched off ( this is wired to the starter battery).

If I take your figure of wet cells self discharging at about 1% to 2% per week, over the period of 14 weeks, the expected loss would be between 14% and 28%. Yet the little dears produced the green eye after the four minute check. It will be another 10 weeks before I return and I expect the batteries to be fine. But it is leaving them year after year for these long periods that made me think about the solar panel.

I have seen an advert for : “The Sunsei SE 400 (6W) can slowly charge large 12V deep-cycle batteries, protecting them through long storage periods. Connects in seconds . . .Includes built-in discharge protection . . . Charge controller, a rough rule is that if the panel puts out 1/60th or less per day of the rated battery capacity, you don't need one. “ Cost £60, not bad, if it saves me having to pay for a crossing a couple of times each winter. But, I fear that if the batteries are holding a full charge, this thing might over charge them, which I understand, sorry, believe, is just as bad as letting them run down.

Edited February 7, 2007 by Lophat Euro

[Gibbo]

I believe the batteries were fully charged, because when I switched on the battery management system, it ran through the checks and the magic eye turned green within four minutes.

 

What is a "battery management system"?

 

It's a very loose term used by various manufacurers to describe *completely* different things. What is it in your case?

 

Gibbo

[Lophat Euro]

What is a "battery management system"? . . .What is it in your case?

 

Gibbo

 

There are three items in the box: an XAlt alternator controller, a Skipper and what the fitting engineer details as, "Merlin 12 volt, 30 Amp charging unit. . . .supply and fit X-split 1 in, 2 BS charge splitter out at same time fitting X-Alt."

Sorry, had to dash up to the attic to get the archive box to find that.

[Gibbo]

There are three items in the box: an XAlt alternator controller, a Skipper and what the fitting engineer details as, "Merlin 12 volt, 30 Amp charging unit. . . .supply and fit X-split 1 in, 2 BS charge splitter out at same time fitting X-Alt."

Sorry, had to dash up to the attic to get the archive box to find that.

 

And which green light did you rely on? The Skipper or the Merlin?

 

Gibbo

[Lophat Euro]

And which green light did you rely on? The Skipper or the Merlin?

 

Gibbo

The Merlin - that's the one with the roaring fan. I gather the Skipper light comes on only when the engine is running.

[Gibbo]

The Merlin - that's the one with the roaring fan. I gather the Skipper light comes on only when the engine is running.

 

Ok. Assuming you have a roughly normal sized battery bank (say 300 to 500Ahrs) the green light on a 30 amp Merlin charger will come on when the batteries reach about 95% charge state. So assuming they were full when you left them this means they have self discharged by somewhere between zero and 5%

 

From this you can get an idea how far they will self discharge over longer periods but remember that it is exponential. ie assume they discharge at 1% per week, from fully charged a 100Ahr battery would lose 1Ahr the first week leaving 99Ahrs. so it would discharge 0.99Ahrs the following week etc etc etc.

 

Gibbo

[Lophat Euro]

Ok. . . they were full when you left them this means they have self discharged by somewhere between zero and 5%

 

Gibbo

Well, that sounds pretty good to me. I reckon "it ain't broke", so no need to fiddle with additions. I am very grateful for the advice.

Edited February 7, 2007 by Lophat Euro

[anhar]

Ok. Assuming you have a roughly normal sized battery bank (say 300 to 500Ahrs) the green light on a 30 amp Merlin charger will come on when the batteries reach about 95% charge state. So assuming they were full when you left them this means they have self discharged by somewhere between zero and 5%

 

From this you can get an idea how far they will self discharge over longer periods but remember that it is exponential. ie assume they discharge at 1% per week, from fully charged a 100Ahr battery would lose 1Ahr the first week leaving 99Ahrs. so it would discharge 0.99Ahrs the following week etc etc etc.

 

Gibbo

Interesting point Gibbo about the geometric rate of discharge. It means that at 1% pw, after a year the battery will be at 59% charged. To reach the critical 50% discharge rate below which we are advised not to go, will take about 68 weeks. At 2% pw the critical 50% is reached after about 34 weeks.

 

regards

Steve

Edited February 7, 2007 by anhar

[Gibbo]

Well, that sounds pretty good to me. I reckon "it ain't broke", so no need to fiddle with additions. I am very grateful for the advice.

 

I'd be inclined to agree in your case. Give them an equalisation charge when you get though to get rid of any sulphate build up. (RTFM)

 

Gibbo

 

To reach the critical 50% discharge rate below which we are advised not to go,

regards

Steve

 

It's more like this.....

 

http://www.smartgauge.co.uk/50percent2.html

 

Gibbo

[anhar]

It's more like this.....

 

http://www.smartgauge.co.uk/50percent2.html

 

Gibbo

Okay, I was just applying simple math. to your above comment stating that batteries discharge at an exponential rate.

 

Steve

Edited February 7, 2007 by anhar

[Gibbo]

Okay, I was just applying simple math. to your above comment stating that batteries discharge at an exponential rate.

 

Steve

 

Right sorry. I was just making that point that the "50% rule" isn't "don't go below this or you'll wreck them". It's more "Do this on average for best battery ecconomy".

 

Gibbo

[Blue_skies]

What make and model of battery would people here recommend (and how much does your recommended battery cost, roughly)? Are there any makes I should avoid?

 

Thanks