Charging three 6 volts batteries

Discussion in 'General Electronics Chat' started by Mothernatura, Aug 2, 2013.

  1. Mothernatura

    Thread Starter New Member

    Dec 26, 2008
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    My PV charge controller cannot charge 6v batteries, so I'm planning to buy three 6v sealed lead acid to get the 18v to replace te battery of the drill wich got fried. I know I could connect two of them in series and charge them without problems, but wat about the third one?

    Can I rotate one already charged and the third one discharged without damaging them? and recharge to fill the third one. I guess once connected in series voltage will level between the two to the average point. Is it recomendable to wait until that leveling ocurrs or it doesn't matther?

    I don't know if my way of explaining what I want was clear.

    Thanks and regards. Paul.

    Thanks.
     
  2. ian field

    Distinguished Member

    Oct 27, 2012
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    If you intend to use the 3 SLA batteries in series, they will last longer if always used and recharged equally in the same series string. Maybe you could charge a surplus car battery from the PV panel, and construct a boost converter for charging the series combination of 3x 6V batteries. There may be appnotes you can download that contain example schematics - Linear Technology (LT) and Maxim are good places to look.
     
  3. paulktreg

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    Jun 2, 2008
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  4. Mothernatura

    Thread Starter New Member

    Dec 26, 2008
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    I guess I'll use an universal notebook charger, it says it gives multiple voltages from 15 to 24 and up to 5A. So if I set it in 21 or 24v it would charge the three in series. The only problem is battery specs says max 1A charge, they may get damaged at 5/3=1.66A, and probably also needs a diode before the batery..

    Perhaps the solar charge controler can handle the three in series, it's a morningstar sunsaver MPPT, and thou specs say it detect batery voltage automatically y don't think 18v is a usual bateria volue to be detected automatically.

    A pity I have to limit to the products available on my local market...
     
  5. ian field

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    Oct 27, 2012
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    Quoting typical figures for 6 cells (12V nominal) - in a fully flat battery; the electrolyte is mostly distilled water with high resistivity, so you need a higher voltage to get things moving (about 19 - 21V), as the acid comes out of the plates and the electrolyte specific gravity increases; the battery is best charged at constant current (1/3 of Ah rating) as the battery recovers the terminal voltage drops to about 10.8V as the SG rises, from there it starts charging - when it gets to about 14.4V you should switch to about 13.6 - 13.8V fixed voltage float charge.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    So are you, but you wouldn't normally describe yourself that way. Battery electrolyte is highly corrosive sulfuric acid. It may be mostly water by weight, but so is fine wine.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    100% pure distilled water is an insulator - look it up.

    Specific gravity is a measure of how much of the acid has reacted with the lead plates, and is also used to measure state of charge.

    A dead flat battery has a low SG and also will be reluctant to take charge current, as charging current reverses the chemical change; the SG rises and the battery is more willing to draw charge current.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Total rubbish.

    A discharged battery is still full of a sulfuric acid solution with a S.G.> 1.1. Toxic and corrosive. It's not a full 37% acid concentration like a charged battery, but it's a long way from your distilled water. It will readily accept as large a charge current as you can reasonably supply.
     
  9. ian field

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    Oct 27, 2012
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    Well I CBA arguing with you - so just carry on stumbling along your dumb ignorant way.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    You need to do more research before you level insults at an expert.

    Depleted battery acid is ~1.1 S.G. This is a 15% solution of sulfuric acid, almost 3M, hardly "distilled water". It's conductivity is 568 mmho/cm at 25°C.

    Charged battery acid is usually less than 1.28 S.G. This is a 37% solution of sulfuric acid. It's conductivity is 787 mmho/cm. That's just 39% higher than the discharged solution.

    In broad strokes, battery acid is battery acid, no matter the charge level. It'll destroy metal, flesh and clothing either way. Discharged battery acid is not remotely "distilled" water anymore than you are.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    You started hurling insults - what does that say about you!

    I stated that 100% pure distilled water is an insulator - anyone who thinks they'll find 100% pure in a battery is clearly *NO EXPERT*.

    It is true that a fully discharged battery with low SG has higher internal resistance and takes a little extra voltage to persuade it to accept the rated charge current.

    But you snipped the main points of my post to try and look clever when you're not.
     
  12. K3CFC

    New Member

    Dec 4, 2012
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    For crying out loud just go buy a new drill. all this hate and discontent is just not worth it.
     
  13. ian field

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    Oct 27, 2012
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    Ah; I see the problem - lack of reading comprehension.

    I'm not the OP.
     
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