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Author Topic: Lister CS SOM battery bank charging  (Read 7388 times)

spencer1885

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Lister CS SOM battery bank charging
« on: July 07, 2008, 09:27:04 PM »
Hi,
I have a Lister SOM 2.5 kva.
  I tried to charge my 48 volt battery bank with a forklift battery charger which draws 1000 watts when plugged into a metre and charges at 40 amps.
When I plug it into my SOM it makes the voltage, which is normally 240 AC, drop to 220 AC and the wattage drops to approx half and it won't charge anywhere near to it's 40 amps.
I have load tested the SOM with the metre connected while running an electric heater and other loads.
I have loaded up to approx 2500 watts and the voltage stays about the same, so I don't see there being a problem with the generator.
Is there something weird going on because I am using a battery charger ( i.e. converting AC into DC), that is making the generator not work correctly?
Any ideas what the problem could be? Or am I making some kind of fundamental mistake?
Any thoughts on using a 48 volt generator running off the engine to charge, instead of a battery charger, as I would like to charge the batteries up as quickly as possible so as not to use unnecessary fuel.
Is there such a generator, which is cheap and easy to find ( e.g. automotive alternator)?

You can see my engine running on YouTube:   www.youtube.com/watch?v=o7YTtMXoPtQ

Any thoughts would be appreciated.
Thanks.

Stan

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Re: Lister CS SOM battery bank charging
« Reply #1 on: July 07, 2008, 09:30:10 PM »
This may be a stupid question, but are you matching your hz?  This being a UK model is putting out 50 hz right?  Just a quick thought.
Stan

spencer1885

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Re: Lister CS SOM battery bank charging
« Reply #2 on: July 07, 2008, 10:25:52 PM »
Hi Stan,
Yeah I am in the Uk and it is 50hz. I have checked that it is putting out 50hz, but not while the battery charger was plugged in. The battery charger should be 50hz as well being a UK forklift battery charger.
When I have put a load on the generator with for instance an electric fan heater, the engine note changes and the generator starts to make a singing noise indicating it is loading up, but when I plug the battery charger in, it isn't loading the generator in any way. Very strange!

mobile_bob

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Re: Lister CS SOM battery bank charging
« Reply #3 on: July 07, 2008, 10:28:17 PM »
you likely have power factor issues with your charger which makes the genset work far harder to deliver what is needed

for instance you want 40amps a 48 volts (lets call it 50volts for easy math) so
40*50 = 2000 watts at unity power factor

my bet is your power factor is probably miserable so you need to generate maybe 3000 watts or more to deliver 2000 watts

then if you factor in efficiency losses you might need to generate close to 4 kwatts to get 2 kwatts charging, and then you are quite overloaded.

to answer your second question

i am not aware of an autmotive alternator that is cheap that can be used out of the box to charge 48 volts, but
there are a number of ways to do so.

a. go to electrodyne and spend big bucks for a 48 volt alternator, or
b. use a 12 volt alternator and some switch gear to charge your battery bank in sections, not a pleasant option but can be done
c. do the same with a 24 volt alternator, or
d. build a custom regulator to control either a 12 or 24 volt alternator so that it can deliver a controlled charge into a 48 volt bank, the alternator
will have to spin faster to do so but it can be done fairly easily.
e. have an alternator rewound to 48 volts, tedious if you do it yourself, or expensive if you have someone else do it.
f. reconnect an automotive alternator stator from the typical delta to wye configuration and then you can use option (d) at a lower rpm.
g. get an st head and reconnect the stator poles via relays so that each pole can be parallel connected which will get you to approx 60volts
but you will need a purpose built regulator to control it as well as a rectifier back to get it to DC for charging.

just a few of my favorite things,,, lalala :)

anyway, i don't believe you have a problem with the SOM, but you likely have efficiency and power factor issues with the charger both of which
will make it nearly impossible to get anywhere near 40amps charging into your 48 volt bank. 20-25 amps is probably more the reality with that setup.

i am relatively certain that you could do much better with an automotive alternator and a purpose built regulator, and maybe reconnect the stator
from delta to wye if necessary to keep the rpm down to a reasonably attainable range.  you might have to go upwards of 6k rpm unmodified, and likely be around
3.5k rpm if reconnect stator to wye.

if you are going to go this route, then step up to a heavy duty truck alternator such as a leece neville or prestolite, an try to find one that is 24 volt to start with
that way the rectifier bridge is likely high enough in PIV rating to handle the 48 volt charging without burnout,,, even if that became an issue the leece nevilles and prestolite alternators have external stator leads that you could remote mount a rectifier bridge to if you found the need for doing so.

bob g


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spencer1885

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Re: Lister CS SOM battery bank charging
« Reply #4 on: July 07, 2008, 10:55:17 PM »
Thanks Bob, I think you are probably right about the power factor.
As for the suggestions....I am afraid I don't quite understand any of it!!!!!! (sorry)
Maybe I could get two 24v alternators and belt them up together to the Lister, and just connect them to the battery bank in two 24v halves, leaving the batteries still connected together as 48v.
What do you think?

solarphil

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Re: Lister CS SOM battery bank charging
« Reply #5 on: July 07, 2008, 11:01:52 PM »
Hi Spencer185

I have successfully used a 24v 120A alternator on a 48v system by altering the field voltage to give the required current.
You would need to get te speed right as well.

However, in my experience auto alternators are not much better than 50% efficiency!!

Phil

spencer1885

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Re: Lister CS SOM battery bank charging
« Reply #6 on: July 07, 2008, 11:20:56 PM »
Hi solarphil
That's probably the way to go.
Why do you say alternators are about 50% efficient?
 Is there a better way to go, without spending lots of money?
If I use a 24v alternator please could I ask you what I need to do in detail, to get it to work the way you did?

oliver90owner

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Re: Lister CS SOM battery bank charging
« Reply #7 on: July 08, 2008, 12:25:33 AM »
I tried to charge my 48 volt battery bank with a forklift battery charger which draws 1000 watts when plugged into a metre and charges at 40 amps.

48 volts at 40 amps is 2kW.  At a charge rate of 2.4 volts per cell, which is the normal controlled voltage for charging you would be using 2.3kW  The innefficiency of transforming and rectifying will increase the load on the supply still further.  So with the genny it  should be well loaded.  If it is not, there must be some measurement inaccuracies somewhere. Ohms law works Ok for straight DC  charging of batteries.

I reckon if you are only using a kilowatt from the mains, to charge at that power input to your batteries , you are on to a good thing. :)

Or am I missing something somewhere?

Regards, RAB

Regards, RAB

ronmar

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Re: Lister CS SOM battery bank charging
« Reply #8 on: July 08, 2008, 01:25:57 AM »
The math dosn't add up.  Watts are watts regardless of the source, and adding in conversion efficiencies only makes things worse.  40A @ 48V is indeed 2KW plus conversion losses.  You are probably just overloading your 2.5KW set and the generator voltage is falling off because of that.

Auto alternators were designed to be small.  because of that they must run at higher RPm to do the job.  In order to get the rotor to survive 6K-7K RPM, thy use a claw rotor design who's generated magnetic field is not as electrically efficient at exciting the stator windings.  They were not designed for efficiency, but to maintain an auto electric system.  i was surprised when I first heard the 50% figure as well, but it is the generally accepted figure.

I think the ideal for charging batteries would be to re-configure a small 3 phase AC generator.  Put a diode rectifyer bank onto the 3 phases then combine these 3 DC outputs(this is exactly what a 3 phase auto alternator does). Then copy an auto regulator circuit but re-configure it for 48V to regulate the generator output to provide the desired charge value. 
PS 6/1 - ST-5.

mobile_bob

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Re: Lister CS SOM battery bank charging
« Reply #9 on: July 08, 2008, 02:21:42 AM »
i am working on a system that uses both an automotive alternator, and transformers with the goal being ~70%
which i believe to be attainable, but not without significant upfront costs both in money and time.

after that excersize i am going to go to the controlled rectifier route to see if i can get similar overall efficiency, which also
is within the realm of possibility, but again with significant costs.

if i were faced with a 48 volt bank i would go with a st 3phase head (as Ronmar suggested) and reconfigure the stator coil groups
and control it with a purpose built 3 step regulator.  such a set up should reach over 80% efficiency in my opinion.

i haven't given much thought to 48volts ...  yet :)
rather i am still thinking about 24 volts as being a better fit for my needs, but that may change over time as well.

the 50% number is based on an average over the rpm range of a typical automotive alternator, the reality is they range from mid 40's to
something on the order of low 60's depending on load and how fast they are spinning, some are better than others which stands to reason.

a prestolite 110-555 has a sweet spot of about 54% efficient in its stock form and is fairly easy to work with, has nice sized ball brgs on both ends
is user serviceable and who's regulator can be bypassed quiet easily. they are priced at about a buck an amp for the 12 volt units and about twice that
for the 24 volt units. overall not a bad unit to work with for modification for other purposes in my opinion. they are made for serious work, where car alternators
usually cannot stand up to continuous high output without loss of longevity.

if it were me i would think long and hard about what it was i really needed and then pick the best approach to get there, no sense in spending time and money on
something that just wont hold up long term.

bob g
otherpower.com, microcogen.info, practicalmachinist.com
(useful forums), utterpower.com for all sorts of diy info

Doug

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Re: Lister CS SOM battery bank charging
« Reply #10 on: July 08, 2008, 04:47:36 AM »
you likely have power factor issues with your charger which makes the genset work far harder to deliver what is needed

for instance you want 40amps a 48 volts (lets call it 50volts for easy math) so
40*50 = 2000 watts at unity power factor

my bet is your power factor is probably miserable so you need to generate maybe 3000 watts or more to deliver 2000 watts

then if you factor in efficiency losses you might need to generate close to 4 kwatts to get 2 kwatts charging, and then you are quite overloaded.


bob g

What Bob is saying is sort of correct but I feel like nit picking.....
When you generate Reactive power you are not generating watts but VARS. We make the distinction because its not real work the generator is doing but rather eneragy that is used to create a magnetic field and it can be compensated for by using Capacitors.

Anyone else got any nits?

Dead on the money on the alternator thing, most autpomotive alternators are make piss poor chargers especiualy if you try and get them to put out the name plate.
The Big chryslers from the early 70s late 60's were smaller in power but very robust if treated well and only asked to put out 20 40 amps contin. They also are easily modifed perform better sped up and have external regulators that can be replaced with smarter ones...
It's a Good Life, If You Don't Weaken

mobile_bob

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Re: Lister CS SOM battery bank charging
« Reply #11 on: July 08, 2008, 05:42:02 AM »
ok Doug:

just for the sake of clarification

if you ask the generator to produce VARs, to provide for reactive current for magnetic circuits
then the generator loses capacity that could have been used to produce useful power?

and i understand that one could use capacitors to provide the reactive power and thus reduce the VAR's the generator has to provide
allowing it to produce more real power (watts), but

the reactive power needed is likely not static but very dynamic, or in other words changes with a battery charger as the battery becomes charged
and the power requirement goes down, much like an induction motor wherein its power factor is worst at no load and best at full load.

i am not sure but i think it is the inverse with a battery charger, better power factor at light load and worse at full load.

most motors are easier to apply capacitors to because the load is generally fairly consistant, but
i am not sure how one could correct the power factor with capacitors on a battery charger, at least over the full working range
without either severely undercorrecting or  possibly over correcting at some point in the load curve.

from what i understand older battery chargers (prior to smart chargers) could have miserable powerfactors downwards of .60 and lower lagging
i wonder if this is not the case with the charger the OP has issues with?

this is a case for better understanding of power factor and its correction, even if it cannot be easily corrected, the lessons learned have broad applicability
for those that generate their own power.

i am certainly no expert on power factor, but i think i know enough about the subject to be dangerous
or maybe ask half way intelligent questions

:)

if you could shed more light on how capacitors could be used to aid in pfc with a transformer/rectifier into a battery bank (shifting load) i am all ears
and more than willing to pull up a chair and learn a more on the subject.

good discussion!

bob g

otherpower.com, microcogen.info, practicalmachinist.com
(useful forums), utterpower.com for all sorts of diy info

solarphil

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Re: Lister CS SOM battery bank charging
« Reply #12 on: July 08, 2008, 01:23:07 PM »
Hi Spencer


>Why do you say alternators are about 50% efficient?

I have come to that conclusion by measurement.
auto alternators require field excitation to provide a magnetic field which the rotating windings cut in order to generate AC
This energy can be considerable part of what is generated.

>Is there a better way to go, without spending lots of money?

You may be able to obtain a permanent magnet alternator which doesn't need extra energy to excite the field.

>If I use a 24v alternator please could I ask you what I need to do in detail, to get it to work the way you did?
Step1
Remove the regulator and isolate the field windings
Step2
Use a low resistance rheostat or make a series resistance for the field winding and feed this circuit separately from the battery.

You can use fixed lengths of Nichrome resistance wire to obtain the field current to produce the required charging current.
You can start with a high resistance which will give low charging current, reduce resistance to increase current.
I can't recall what resistance I used for the 48v unit but for 24v charging, I used around 10ohms for 60A

I think I was able to obtain around 60A from a 9HP Honda motor into a 48v battery bank.

The efficiency is calculated from measuring the power out divided by the power in.

For example:
60A x 56 volts = 3360 watts output
Input = 9HP = 9 x 746 =6714 watts
efficiency =3360/6714 = 50%!!

I suspect you may not be able to obtain 40A @ 55 to 60v to fully charge your battery set - you can adjust the field resistance until you are happy that the motor is able to cope!!

Have fun!!

Phil




Doug

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Re: Lister CS SOM battery bank charging
« Reply #13 on: July 08, 2008, 04:29:40 PM »
Bob the reactive power stays about the same from no load to full load on motors and transformers but the real power consumed by the machines changes from load to full load. The apparent power ( VA ) consumed seems to change as does the power factor but thats because the ratio of real power to reactive changes.

Am I making sence?

Yes the old bigger battery charges had bad PF and don't forget old welders too.
Anything with a transformer in it with little of no load on will realy have a sucky PF unless its corrected for with caps. This is easy to do if you have a clamp on amp meter just add caps at no load untill you bring the current down as much as possible ( too much correction will make current rise again so you know when its time to stop adding )
It's a Good Life, If You Don't Weaken

mobile_bob

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Re: Lister CS SOM battery bank charging
« Reply #14 on: July 08, 2008, 04:42:25 PM »
Doug:

i have several references that state that lightly loaded or unloaded induction motors exhibit their poorest powerfactors and it shifts as the load increases
to higher power factor, in some cases the shift is quite dramatic.

i can see pfc with capacitors on a motor that is under a relatively steady load, but even then it would probably need to be corrected to not much over .95lagging
so that it would not shift to overprocted at full load. a compromise i guess.

from everything i have read on the subject one would need some form of dynamic correction for something like a battery charger, where the load shifts significantly

i guess i will go back and dig up the books, been a while and his gives me an excuse to do a bit more reading.

bob g

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