Battery Charging Question

[taco6513]

This is a radio question from my AUXCOMM class. The scenario was a cell site with 48V power system

has lost it's normal power supply and batter charging ability. Also 2 Public safety repeaters are on this

battery source and battery power will run out in 4 to 6 hours.

I have drawn a picture of a possible solution. 4 12 volt batteries in series to make 48V and 4 12 volt

batter chargers to recharge the 12 volt batteries in series.

Will this work?

Repairs will be made in 48 hours.

Thank you for looking.

WRCW870

Lee.

[BoxCar]

I don't believe i will work as drawn. You need to charge the batteries with a 48V+ source. Now, could the chargers be in series, I don't know but our resident P.E. Mr Shannon may be able to answer.

[gortex2]

So been here done that. As said 48v charger is ideal, but not required. It can be done with a 12v charger one battery at a time. We have charged strings sitting in a warehouse like this. One battery each day. While I've never tried to charge all 4 at once as long as the charger is truly isolated from ground I would think it would work. The issue is many of the cheap old charger the 12V negative lead it grounded to the chassis which is also plugged into the wall. 

[WRYZ926]

Best is to charge the batteries all together at 48V. That is how every 24V, 36V, or 48V system I have worked with is set up for charging to include engine driven alternators and generators. Look at all the electric golf cart chargers for an example. They charge to the correct voltage for the system versus the individual batteries.

[axorlov]

How the batteries connected is not a series connection, it is an explosion device. Where "48VDC" is written will in fact show 12V for a very brief period. Every battery is shorted via the other battery in the pair, current will be high and will destroy the batteries in a matter of seconds.

[axorlov]

This is what you want:

In this case it is possible to use four chargers, but with the load off, because the output will be higher than 48V with the chargers ON.

 

 

[taco6513]

AXORLOV, thank you for correcting my drawing. This would be a short term solution.

48 hours. This is a better and correct drawing.

Thanks

[SteveShannon]

2 hours ago, gortex2 said:

So been here done that. As said 48v charger is ideal, but not required. It can be done with a 12v charger one battery at a time. We have charged strings sitting in a warehouse like this. One battery each day. While I've never tried to charge all 4 at once as long as the charger is truly isolated from ground I would think it would work. The issue is many of the cheap old charger the 12V negative lead it grounded to the chassis which is also plugged into the wall. 

I’m not an expert on battery charging circuits, but what I do know agrees completely with this.

[axorlov]

Gortex actually pointed out important consideration: chargers must be isolated from the ground and from each other. Is it worth the hassle? Another consideration: lead-acid batteries are charged with 13.5-13.8V, your load will see about 54-56V instead of 48V when chargers are ON.

[Lscott]

The problem with series connected batteries is “balancing” them. Slightly different battery voltages can indicate a difference in the state of charge. Good battery systems use a “BMS” to keep all of the batteries well balanced during charging.

The article at this link goes into much more detail about the various issues.

https://www.allaboutcircuits.com/technical-articles/introduction-to-battery-management-systems/

 

[SteveShannon]

9 hours ago, Lscott said:

The problem with series connected batteries is “balancing” them. Slightly different battery voltages can indicate a difference in the state of charge. Good battery systems use a “BMS” to keep all of the batteries well balanced during charging.

The article at this link goes into much more detail about the various issues.

https://www.allaboutcircuits.com/technical-articles/introduction-to-battery-management-systems/

 

Good article and I agree that a BMS is necessary for high energy density batteries like Li-ion, LiFePo, or Lipo batteries, but in my admittedly shallow experience (a customer of the utility telecommunications department and for my wife’s electric wheelchair), if the OP is charging lead acid batteries, a BMS is not necessary.  For utility telecommunication sites (I was in a different department that depended on them, so again, I’m not an SME) they simply used strings of ten lead acid batteries in glass or plastic trays for a -125 vDC system.  

Hopefully, by now, the OP has already worked it out.

[WRKC935]

So here's the issue with using "battery chargers" referring to the normally available 12 volt car battery chargers that would be readily available.  They are non-regulated and non-filtered.  Due to that the an AC hum would be superimposed on the DC power and would show up it the audio signals of the radios.

Proper way's.

First is a site generator.  Most current 48 volt rectifier systems are modular. Meaning they are not one huge DC supply but a number of independent supplied in parallel that charge the batteries and power the connected equipment.  So a full failure of that equipment is very rare as opposed to a utility power fail that renders them inoperable due to no input power.  Any properly constructed cell or communications site will have in place a manner to connect a large generator that will run the site during a grid down event.

A medium sized (30 to 60KW) tow behind generator and the correct cable would be the correct way to run the site.  ANd you are going to need something larger to run a bunch of 200 amp battery chargers anyway if you tried to do it the way shown here. 

Second way is a purpose built 48 volt generator.  The phone company has had these for years.  They look like a standard large portable generator with an 18 to 20 HP motor but the output is 48 volts and not 120 / 240 volts.  They are built specifically to power a 48 volt battery plant during maintenance or replacement of a 48 volt battery plant rectifier. 

Something of this nature could be constructed with rewiring a couple alternators with external regulators and running them in parallel to produce the required current level needed for such a thing.  Contact your local telephone company or industrial generator dealer for more information.

 

Lastly, and the way to NOT do it unless the chips were down and there was no other way.

Similar to the manner that was shown but with switching power supplies of a current rating that meet's or exceeds the load at the site, but with a clean output and not a bunch of noisy battery chargers.  Still need a generator if the grid is down.  But a better way is find a proper 48 volt rectifier and a generator, put it all in an enclosed trailer and drive the whole thing out to the site.  Remember that you are going to need wire to get to the power distribution breakers to feed the system.  SO if you are serious about doing all this,  you need to prepare ahead of time and not think that you are just going to wing it during an actual event.

 

[WRKC935]

Oh, BTW.  If you decide to try independant supplies.  You need to verify that the ground is ISOLATED.  Meaning the battery side output is NOT connected tot he ground pin on he power cord.  If it's connected on two or more of the chargers, the 12 volt power in the batteries will back flow from the adjacent battery back through the power cord, into the building ground and then into the ground of a battery effectively creating a dead short.  With a set of even partly charged batteries, you will cause a fire at that point and things will get out of hand quickly.

 

[Lscott]

2 hours ago, Sshannon said:

Good article and I agree that a BMS is necessary for high energy density batteries like Li-ion, LiFePo, or Lipo batteries, but in my admittedly shallow experience (a customer of the utility telecommunications department and for my wife’s electric wheelchair), if the OP is charging lead acid batteries, a BMS is not necessary.  For utility telecommunication sites (I was in a different department that depended on them, so again, I’m not an SME) they simply used strings of ten lead acid batteries in glass or plastic trays for a -125 vDC system.  

Hopefully, by now, the OP has already worked it out.

Lead Acid batteries can still be unbalanced. The way the batteries, and cells in each one, are balanced is by over charging so the cells gas. That’s fine with liquid filled types. However you can’t do that with sealed types since you can’t add distilled water to bring the electrolyte level back up. If you let the sealed types gas they will eventually be destroyed.

Also the charge voltage has to change with temperature. Good LA  battery chargers have a temperature sensor you attach to the out side. 
 

https://batteryuniversity.com/article/bu-403-charging-lead-acid 

If you look at the data sheets for the sealed types you find there is a maximum charge rate specified which keeps the cells from gassing during charging.

I’ve seen more than once people selling solar power generators at swaps with a 7 amp-hour gell cell, max charge current is typically 2 amps, in the box being charged by a 100 watt panel that outputs around 8 amps. Those batteries won’t last long.

[WRKC935]

Thought on this some more. 

Couple more considerations with regards to the battery chargers.  A 48 volt plant is actually -48 volt supply.  Meaning the positive side of the end of the battery string is connected to the ground or 'return' in battery plant speak.  So again, if the output of the battery charger / power supply HAS to be isolated.  So again, you need to be aware of what you are doing and what you are working with if you are going to mess with this. 

But again, 48 volt battery plants have serious current producing abilities and doing something incorrectly when it comes to this level of energy can have disastrous results.