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 Post subject: 110V DC Power Plant
PostPosted: Dec Thu 06, 2018 11:36 pm 
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Location: Oxford, MI
I have a number of radios that will operate off of AC or DC mains. I also recently aquired a number of 12V 5Ah sealed lead acid batteries that were given to me by my work, they came from decommissioned APC units but test fine for capacity and voltage.

My thoughts are to take 9 of these and run them in series for a 110V DC power plant for rechargeable noise free operation of radios off the grid. I plan to have an isolation transformer followed by a bridge rectifier and voltage dropping resistor to set the charging voltage for float charging. This would be coupled with a 120V relay and fused outlet so that when power was being drawn from the battery bank it would be taken off charge.

Has anyone experience with this? Any suggestions?

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 Post subject: Re: 110V DC Power Plant
PostPosted: Dec Fri 07, 2018 3:00 am 
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Location: Albuquerque, NM 87123
Should work, but since the batteries are used they will not charge at the same rate.


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 Post subject: Re: 110V DC Power Plant
PostPosted: Dec Fri 07, 2018 3:45 am 
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Correct. The charge and discharge imbalance will only get worse the more times they cycle. It was an issue with as few as 3 or 4 batteries in my electric motorcycles. I ended up using separate small chargers. Not feasible for your situation. There are equalizing solutions but its a hassle to have to deal with it.


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 Post subject: Re: 110V DC Power Plant
PostPosted: Dec Fri 07, 2018 5:26 am 
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Hmm, I hadn't considered that charging imbalance could be an issue. These batteries came out of large 96V battery backup systems so they were charged and discharged in series for most of their lives, and I don't believe that they are more than a year old.

It would be possible to employ a number of DPDT switches in order to create a series parallel arrangement, so that they could be charged in parallel from a single higher current output 12V charger, and then switched into series for the higher voltage, but that may be too mechanically complex.

I may have to rethink this.

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 Post subject: Re: 110V DC Power Plant
PostPosted: Dec Fri 07, 2018 3:12 pm 
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Location: Equinunk PA 18417
It is current that does the charging, isn't it? It seems to me a trickle charge approach, constant voltage with low current, would be excellent for a group of series cells. Nearly all vehicle batteries are series cells and this consideration is nothing new. Current through all cells in series MUST be the same and so all eventually will be re-charged to their top conditions. Float charging, where the charge current is removed from time tot time, may not be a good idea without some examination of individual cell conditions. The OP has old batteries destined for recycle bin but unless he needs to draw big currents with a requirement for rapid recharge, I don't see a problem!
I'm no expert here so please correct me.

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 Post subject: Re: 110V DC Power Plant
PostPosted: Dec Fri 07, 2018 3:37 pm 
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Location: Albion, CA, USA, 95410
Johnnysan wrote:
Should work, but since the batteries are used they will not charge at the same rate.


Eventually, sure. I have 16 golf cart batteries in a series/parallel array for my home power (I live off the grid). I give them an equalizing charge every few months. They are almost 12 years old and on their last legs but still do the job.

I wouldn't worry about it too much. If you like you can switch battery positions and equalize periodically.


Last edited by madlabs on Dec Fri 07, 2018 3:59 pm, edited 1 time in total.

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 Post subject: Re: 110V DC Power Plant
PostPosted: Dec Fri 07, 2018 3:44 pm 
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Location: Albuquerque, NM 87123
If you open up most rechargeable batteries that are more than 1.2 volts (ie. batteries in series) that have failed you will probably find that only one or two of the cells have failed; the rest will test OK. The good ones may have a few months or years of useful life left in them.

Will they charge equally? NO! Especially if you put them in series.


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 Post subject: Re: 110V DC Power Plant
PostPosted: Dec Fri 07, 2018 4:14 pm 
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Location: Albion, CA, USA, 95410
Yes, the most common failure mode of a battery is one cell bites the dust first. And it is true that the more batteries you string in series the less equal the cell voltages will be. This is generally dealt with by keeping the interconnect wires of identical length, good contact maintenance and periodic equalizing charges. In high current systems like off grid power it is a bigger factor but still manageable. In this case with low currents and the ability to move the batteries around in the string I wouldn't worry about it. For sure I wouldn't add switches and wire to try to charge them more equally, that will add more unequal resistance in the charge circuit and increase the likelihood of imbalance.

Use 'em, enjoy 'em and don't worry about it.


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 Post subject: Re: 110V DC Power Plant
PostPosted: Dec Fri 07, 2018 4:17 pm 
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Location: S. Dartmouth MA 02748-1225 USA
madlabs wrote:
Johnnysan wrote:
Should work, but since the batteries are used they will not charge at the same rate.
Eventually, sure. I have 16 golf cart batteries in a series/parallel array for my home power (I live off the grid). I give them an equalizing charge every few months. They are almost 12 years old and on their last legs but still do the job. I wouldn't worry about it too much. If you like you can switch battery positions and equalize periodically.
O.T.
Facinating.. I once had an electric car with 16 golfcart batteries, 225ah, FWIR about 125# each, converted Renault LeCar. I drove with an ammeter, accelerate pinned 1k amps fall back to 100 amps cruse, 600 amps average hill. All when fresh charged. Usually got 25 miles in first summer by 2nd summer 20, when winter came couldn't make my commute, after 17k miles batteries shot. I traded the vehicle for a gas station wagon... Never concerned about equalizing, added an extra 15 minutes time on the charger until all cells gassing freely...

OTOH I salvaged a pair of 6volt NIFE batteries from an emergency lighting system in '79. Added distilled water, washed off crystal growths. I still use them after 39 years for my 20's radios, they were some 15 yo when taken out of service, so I estimate they are 50 y.o. Edison still has a great Idea.

Chas

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