# 120v DC @ 20A power - using 12v batteries/chargers?

Discussion in 'General Electronics Chat' started by DMahalko, Jun 10, 2012.

1. ### DMahalko Thread Starter Senior Member

Oct 5, 2008
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I want to build a 120v DC power supply with 20 amps power capability for up to about an hour (2400 amp/hours).

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120vDC nominal (134v DC fully charged) is easy enough to do with ten 12v lead-acid batteries in series, but finding 120v DC lead-acid "battery string" chargers is extremely difficult and uncommon.

Is there any problem with just using commonly available 12v batteries and 12v chargers, and wiring them together in a series/parallel circuit?

Basically each charger is connected across the terminals of a single battery, so each battery can be charged separately.

Then the positive terminal of one battery/charger group is connected to the negative terminal of the next group, until we have ten in a row and 120v DC nominal output.

For safety and short protection, I would put a 20 amp circuit breaker or fuse between each charger/battery group, so ten 20 amp breakers, limiting the overall wattage to 2400 watts at 120v DC. To remove batteries or chargers for service, any of the breakers can be opened to disable the string power.

The main safety concern would be just covering up or insulating the tops of the battery terminals so there isn't a high-voltage risk of accidentally bridging across different group terminals, while working on the battery string.

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Should there also be separate breakers/fusing for each of the battery-to-charger connections, like this?

If the chargers have enough supply capacity, they can help supply the 120v DC output while also charging.

- Dale Mahalko

Last edited: Jun 10, 2012
2. ### ronv AAC Fanatic!

Nov 12, 2008
3,393
1,451
Did you really mean 2400 amp hours at 120 volts or 2400 watts?
Wait I see 1 hour. So 20 AH at 120 volts.
So lets say you use 50 AH batteries and 5 amp chargers that should work.
If you buy good chargers they should protect themselves so shouldn't need separate breakers.
Be very careful. There is a reason Golf carts are usually only 48 volts.

Last edited: Jun 10, 2012
3. ### DMahalko Thread Starter Senior Member

Oct 5, 2008
180
14
After doing some more research it appears to be important that the DC output of each charger be electrically isolated from AC electrical grounding in the charger device.

If not isolated, there will be a short between chargers of the string, via the grounding connection of each 12v charger circuit.

Trying to find an "isolated 12v DC charger" is even more fun than trying to find a "120v DC charger". Maybe they are, maybe they aren't. It was never important to document that, so common charger device specs say nothing one way or the other.

Though, this issue could be solved using AC isolation transformers and disconnecting the AC ground of each charger. These transformers don't need to be huge, as a 2 amp transformer is enough for about 20a @ 12v charging.

- Dale Mahalko

4. ### DMahalko Thread Starter Senior Member

Oct 5, 2008
180
14
Pardon me while I continue this discussion with myself.

(I realize that this topic of 120v DC power is ridiculously obscure, but the forum is crawled by Google, so perhaps my continuing documentation here will help someone else answer similar questions in the future. Also, hello to any future employers searching me. )

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I have determined that the correct type of charger to search for is simply an "isolated DC charger" -- no need to mention voltage, as there can be a wide range of voltages for isolated chargers.

This actually brings up some really specific results that are very on-target for this application:

Quick Charge Corporation - Multi-Bank Chargers
http://www.quickcharge.com/multi%20bank%20chargers.htm

From their info page:
These chargers can be mounted on board equipment, or mounted on a shelf or wall. No need for factory programming, each circuit is user programmable via internal dip switch to charge deep cycle wet cells, starting wet cells, gels, AGM's, and lithium Ion. Each circuit is current limited, and polarity protected. As an example, they can be connected to each battery of an electric vehicle, and since each battery is charged independent of the other, battery life can increase by at least 25% over a conventional series charger. In another example, you could attach multiple modules to one or more batteries to increase the charge rate in 10 amp multiples.

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Charging each series-wired cell separately has added features for photovoltaic and electric vehicle applications since some cells charge quicker than others as the series string ages.

By charging each one with its own dedicated isolated charger, the batteries that need a slower charge get the attention they need, without overcharging and outgassing the ones that charged quickly.

- Dale Mahalko

5. ### DMahalko Thread Starter Senior Member

Oct 5, 2008
180
14
Also from looking around further on this website, it appears they may have the 120v nominal DC power supply I was trying to find in the first place, except they call it a "series string variable-voltage battery charger".

Because what use is there for 120v DC for anything else but battery charging?

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Quick Charge - Series Multi-Charger
http://www.quickcharge.com/series charger.htm

Not only does this company have a 120v DC series-string battery charger, but they can also do 240v DC. Fun!

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The instructions for the 240v DC version are quite entertaining:

QPAX36-240V/6A, MULTI SERIES Battery Charger
http://www.quickcharge.com/Images and PDF's/Series/QPAX36 series inst.pdf

INTRODUCTION:
The Series Multi charger is designed to charge 18 - 120 cells in series at a 1 to 6 amp rate. The charge rate is controlled by one infinite adjustable control knob.

WARNING:
THIS CHARGER HAS THE POTENTIAL TO PRODUCE UP TO 300 VOLTS DC WHICH CAN BE LETHAL. EXERCISE EXTREME CAUTION WHEN HANDLING LEADS. ALWAYS TURN POWER OFF FIRST. THE BATTERY STRING ITSELF CAN CAUSE A SEVERE SHOCK OR ELECTROCUTION IF BODY PARTS MAKE CONTACT WITH THE BEGINNING AND END OF THE STRING.

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Though, using this directly as a 120v/240v DC power supply may not work, because they do voltage sensing at turn-on and don't supply charge current unless a battery string is detected:

TROUBLESHOOTING:
NO POWER IS PRESENT ACROSS THE DC LEADS WHEN A VOLT METER IS CONNECTED
Good. The charger will not turn on until the clamps are connected to the battery.

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In general the "fix" for this appears to be to simply build a tiny string of 10 or 20 12-volt lead acid batteries of the smallest amp-hour capacity possible, to use as a "tickler" to get the charger outputting current.

The adjustable charger voltage output would be set at or just slightly below the string voltage so that the charger doesn't actually charge the tiny battery string, but will supply current when connected to an external device.

- Dale Mahalko

6. ### collo New Member

Oct 5, 2014
2
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Last edited: Oct 5, 2014
7. ### collo New Member

Oct 5, 2014
2
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Hi Dale, many thanks for your posts. I have a 10 x 12vdc x 200amp/hr battery bank in series attached to 2 x 2ooowatt x 120 vac wind turbines. Regretfully the Chinese manufacturers supplied us incorrect wind requirement specs, so we ended up with 2 turbines that are totaly useless in our location. Consequently we have a battery bank that is now flat and needs a decent battery charger. This company seems to have what we need so will give them a try. Mind you your idea with multiple chargers is what I was considering, so depending on dollars and charge capacity I may still go that way.We didnt want to run a genset 24/7, hence our hope to use wind and some solar, I think that may be a bust at this point.