Charging Batteries (Efficiently) with a Diesel Generator

[Usman]

After thoroughly going through the past discussions on this subject on this board, I am presenting here the uncovered part of this subject, which is efficiency.

I have a 48V/5KW Wind/PV Hybrid system in a remote location using a Tri-Star 60 charge controller for the PV and direct connection of the Wind Generator to the batteries via a rectifier.

I am investigating the efficiency of an off-the-shelf electric start diesel generator to charge the battery pack when daily target of a certain KWH production isn't fulfilled by the wind & PV.

Here we have two different ways of doing that:

1. -Direct Connection (Cheaper): Directly connecting the Diesel Generator to the battery pack via a rectifier.
   
2. -Indirect Connection (Expensive): Here, an off-the-shelf battery charger is connected to the output sockets on the Generator and batteries get charged via this battery charger.
   
   

The main efficiency comparison here: which set-up would run the Diesel Generator at its optimum loading level.

In other words, making sure if the batteries load the generator up to it's optimum level

VS

If the Battery charger is able to make the Diesel Generator turn at it's optimum level. Here we have to account for the efficiency losses within the conversion process.

The optimum loading on any Diesel Generator is around 85% of it's Name plate rating, running it at any lower or higher rate would mean fuel inefficiency and/or wear/tear of the generator, resulting in increased servicing visits.

In addition to that:

The battery charger adds additional cost and introduces another component to the system which would mean comparatively less reliability.

On the other hand, the batteries would have lower impedance on the Diesel Generator (not sure on that one but mist probably) and if that's the case,  the load on the Diesel Generator would be around e.g. 30-40% (not sure on thison too) depending upon the SOC of the batteries.

Any input would be appreciated.

Thanks.  

[dnix71]

If your batteries are expensive or hard to replace to will get you in trouble if they fail without warning, then the smart thing to do is worry less about efficiency and make sure the batteries are properly charged. Lead acid batteries aren't the most 'efficient' choice for hybrid cars, for example, because the rate of charge must decline as the charge level (voltage rises) or damage will occur from overheating, and loss of water through electrolysis.

NiH and Lithium batteries have more stable voltage from empty to full, so the current doesn't have to taper off as the percent charge rises.

If you are going to charge batteries that may be near full charge, then a trickle charge will be more efficient and less likely to damage your batteries.

It might be cheaper and better to get a small generator and use an alternator on it and just let it idle. It isn't good to run any gas/diesel engine for a short time at high load anyway. Engines need a certain amount of warm up time to drive out any condensation in the oil and any fuel blowby that would dilute the oil.

[Usman]

Thanks for your input.

The Diesel Generator is configured to be switched ON for three-four hours, when a voltage sensor activates the generator upon sensing a DOD of 80%, so that the Diesel Generator only charges the battery pack up to 50% SOC, leaving a room for the wind turbine & solar array to charge up the rest as when they are available, until the voltage sensor triggers the 80% DOD threshold again.

So, there is no chance of overcharging by the Diesel Generator, unless both the timer and the Voltage sensor fail!

In case of Direct Battery Connection with the Diesel Generator, the Generator would be loaded (up to what level???) until 50% SOC is reached (presumably!).

[bob g]

check out the following

amplepower.com

rolls or surrette battery site

sandia labs

all of which support the 50/80 regime

which will result in the best efficiency in charging your batteries

going to 80% depth of discharge will dramatically reduce the life of your batteries

battery lifespan should be part of your efficiency calculations in my opinion.

if you are going the diesel route

get a small single cylinder (4-5hp) a good hot rated alternator, and a very good 3 step controller. otherwise you will be wasting fuel and iron in my opinion.

i among others here and elsewhere could write a book on this topic, there is alot of info out there if you do your homework.

things i would not do

i would not run a diesel generator to power a battery charger (even if it were a high tech charger) your efficiency will be lower than with what i described.

i would not run an engine/alternator/charge controller to charge from 80 to 100%

but once every week to ten days, otherwise you will spend alot of dollars on fuel needlessly.

just a few of many thoughts on this topic

bob g

[97fishmt]

Hi Usman,

Are you missing something here?  Or maybe it's unclear what you are

trying to do.  Is your generator set up to put out about 60 volts

for direct connection?  Then just go through the tri-star.  Why would

you want to hook up another battery charger?  

I use a high amperage alternator coupled to a honda  I/C motor for

low voltage charging.  It has been figured out to be about the best

way to go.    

[Usman]

Thanks,

The ample power is a good choice but only at a price, around $13,000.

Matching a high amperage alternator to an engine is a tricky thing and needs a lot of testing ( with differnt pulley dia.s )before it could be optimized.

I was thinking of an automotive type engine, perhaps an ISUZU Diesel from older models, from a scrap yard, and replacing the existing belt driven alternator with a heavier one. I presume it could be run at an idle speed 1000 rpm, if a larger DRIVER Pulley was retrofited onto it.

But here again, we need a lot of testing, and need to make sure that the fuel consumption is minimal and the engine doesn't run over or underloaded.

[Usman]

The alternator's output is stepped down using an iron-core AC Transformer, then run through a rectifier to output DC current to charge batteries.

The Tristar is hooked up to the Solar Array, but it is also tunned to function as a diversion charge controller, i.e. diverting access power when preset SOC is achieved.

[bob g]

dig a bit deeper on the amplepower site

and you will find a ton of info on the proper charging of batteries

forget their engine driven genset, yes it is very spendy

if i were to recommend a setup

i might go for a r175 changfa coupled to a 175-200amp leece neville

or a 110-555jho prestolite 160amp

derated to around 80-100amps

spin either to around 4000rpm alternator speed

and attach a balmar, xantrex or amplepower controller

the controllers are programmable to match the alternator to the available hp

programmable to the battery type and size, etc, etc.

how big of a battery bank are you working with? what voltage?

bob g

[Usman]

The Battery Pack is around 25kwh, lead Acid type, 48VDC.

Can you further expalin what these controller can do, and how much they would cost? This matching seems to be a tricky job and, why not simply run the alternator without this controller?

[97fishmt]

Hello Usman,

You are going large I see.  I would try to go with at least a 1800 RPM

diesel and a small one.  You are probably going to be faced with 6kw or

10kw to 20kw units.  If you are just concerned about electric start battery

charging then propane small 5-10hp engines would be far better for their

fuel consumption than a large genset. But if you will use a large genset

alongside charging such as doing the laundry and charging or running the

well pump wile charging then I would probably go with the diesel.  The fuel

could also make a difference.  If you get into bio-diesel? I know that direction

inspires me.

[Usman]

Lister engines are my favourite, already covered on this board. But I haven't got any access to those.

Thats the reason for hinking about a calvaged diesel engine of a Isuzu, as it would be probably running at around 4-6 hp at idle speed (Just a rough guess). And also parts are widely available.

Any makes or model that could also run on Biodiesel or vegoil, in addition to fulfilling these requirements?

(Hopefully that doen't change the core topic of this posting!)

[97fishmt]

I've gone this direction with gas car engines and I thought

it would be great at first, to have large power access and

electric start from comfort, but, I found out it does not take

a lot of horse power to create energy to charge batteries. I use

a 5.5hp brand new Honda vertical shaft internal combustion engine

to turn a 48 volt Leese Neville alternator, with a adjustable field

control. This all operates at a little over idle for the 5.5 motor,

putting out as much amperage  as I need. This is not automatic, this

is for when needed. I think you will find it hard to answer what

you want to do. Unattended?  

Good Luck and Happy Trails!

[Usman]

Not sure what you mean by over-expecting from automotive gas engines, but an automotive diesel engine's turn at around 600-800rpm when idle, and should be putting out around 4-5hp at least at that speed, and that as well at a superb fuel consumption, so I still find that an "automotive diesel engine conversion" a very attractive technical proposition, but again, no verdict could be made until after testing.

Thanks for your input.

[97fishmt]

O.K. I just thought you were going the cheep route.  I too would like a nice diesel

genset and all that goes with it. I know lots of people here do.  Just having your inverter kick on the generator to charge because of insufficient charge may not be

what you are after if you are not there.  You did not say if this was a residence.

You did say it was remote.  For a large system like you are describing get the

diesel genset and the fuel delivery to with it and you will better off than

most people in the city, if you enjoy it , but it will cost you quite a bit per

kwh of diesel compared to the grid.

[Flux]

I haven read all this and the comments but basically I think you have contradicted your own intention in the question.

The most efficient way is with a directly connected generator and this generator would need to be compatible with battery charging requirements. I couldn't see you finding such a thing commercially suitable for use at 48v.

"The alternator's output is stepped down using an iron-core AC Transformer, then run through a rectifier to output DC current to charge batteries."

That answers your own question, that is not direct connection, you have included a battery charger even if simple and home built. Once you go the battery charger route then your original question goes out the window. The new issue is what is the most efficient battery charger.

Beyond this the issue comes down to so many factors that I can't be bothered to wade through it. The high tech battery chargers are primarily designed for mains use and their main purpose is to look after the batteries. On mains they take what is needed to do it.

Running these things from generators alters the equation where you are looking at fuel economy load etc and in most cases they are expensive and far from ideal but they do give ideal charging characteristics.

Your transformer and rectifier if using a suitable transformer will be as good as the expensive high tech things but will not be as good as a direct 3 phase alternator rectified and set up for battery charging directly at 48v.

Single phase is dreadful for battery charging.

You will have so much advice that you will be no wiser in the end.

Flux

[Usman]

Thanks Flux,

Great explanation!

The reason for the inefficient iron core generator, mainly not to mess up with a readily available 220V single phase generator, by simply running a wire from it's output socket to a 1:4 or 1:3 transformer, so that when the resulting single phase AC passes through the rectifier, it gets within a reasonably efficient charging frame.

Yes, single phase & iron core transformer, both add to inefficiencies. So, what about directly connecting the high A/C line (220VAC) into the battery pack (after rectification)? Wouldn't that be comparatively efficient?

I know that a 3-phase PMA turning within the 50-60VDC Voltage frame is the most efficient way to go, but that would need some re-engineering work to an off-the-shelf generator.

It is preferable to either go for no retrofitting at all, or designing a completely custom-designed & built engine/generator.

To comply with the second route, any comments on the automotive diesel engine's retrofitting with the right PMA running at idle speed or just above idling speed! Wouldn't that be most efficient from both aspects i.e. the fuel consumption as well as battery charging? Also easier maintenance, widely available parts, cheaper, etc.....

Thanks.

[spinningmagnets]

I agree on desireability of Lister...

Late 1970's VW's and Audi's sometimes had a non-turbo 4-cyl diesel, later models had the "TDI" turbo-diesel.

I occasionally see early 1980's Mercedes diesels. 240D is the 4-cyl, 300D is the 5-cyl. When the automatic transmission wears out at ~250K miles, they are often crushed, even though the engine still runs fine. Uses gears instead of timing chain.

Keep and eye out for industrial equipment and military surplus auctions. I've seen water pumps, concrete cutters, etc, with a small diesel...

[Usman]

Any idea on their hp rating @ idle speed?

[jacobs]

I use a 7500 watt Kohler generator direct wired through a bridge rectifier to charge my 144 volt battery bank. It's wired from the 240 volt output on the generator, through a bridge rectifier, and then to the batteries. To match the generator voltage to the battery voltage, I installed a spring on the engine governor to reduce the engine speed (about 50%) and voltage. I've been using this method for about 15 years and it's been totally trouble free. I personally wouldn't use any other method. KISS!

[scottsAI]

Usman,

efficiency

In the name of efficiency be careful of just what efficiency your going after.

The system you propose is a balance of many efficiency's.

Battery cost

Battery Life.

Fuel cost.

System cost etc.

Fuel efficiency = Energy efficiency charging a battery best below 80% SOC (which you know)

Battery life is best when SOC is above 80% even better above 90%...

Battery cost efficiency means maxing out battery capacity (how low in SOC (20%)).

Battery life is best when minimal capacity is used. (Battery charge/discharge cycles)

Both effects to Battery life are controlled by a condition know as sulfating.

Sulfating

All about SOC over time spent with low SOC and how long since last equalizing charge. Timer is assumed to reset after an equalizing charge.

Is Your daily energy use predictable?

By knowing at end of day the battery is going to sit until morning for sun, during the last hours of battery use charge it to 50% to minimize time at low SOC.

The more time spent with low SOC the more frequent an equalizing charge is needed to max out battery life.

Battery can be charged at 1C rate, only do this after studying the dangers of doing so.

Books have been written on the care of lead acid batteries, this only hits on the high points.

Be careful of what you read, many urban legends out there on the care and feeding of batteries, much is BS. Find real sources backed by tested data to draw any conclusions from. In others words do not take my word for this! Sorry but the price is a system that does not meet your needs... which can you afford?

The other balance is system design of wind / solar, are they sized to supply needs or will the generator be supply a daily boost to the system?

Solar panels have a present cost vs fuel a future cost, all part of the balance of system design.

Have fun,

Scott.

[spinningmagnets]

Sorry, I don't know the idle HP. I've never done this, but I've seen people who have, so I know its do-able.

I've seen a few Isuzus around, but VW and Mercedes seem to be the most common 4-cyl's, here's a more complete list:

http://www.grassolean.com/index.html?textFile=dieselcars

If you're only idling, turbo-diesel's will run without the turbo, and if you get a "near-free" engine thats too big, a 4-cylinder can be run on two cylinders. I've just been collecting information for my retirement, so I haven't done this myself, but I have seen it done.

IMHO, a properly cared for Lister should last for decades, and is well worth the cost.

[bob g]

first of all these controller are typically for 12 and 24, i am not aware of 48 volt but they may be available as well, or one of the others might be made to work

with a voltage divider on the sense line.

a typical controller does the following

three step chargeing, bulk, absorption, and float, and some have manual equalization

they have a ramp up feature which brings the alternator up online slowly

saving belt wear

they have a feature to tailor the output to the available hp you have driving it.

they have alternator temp sensors which save cooking the alternator

they have battery temp sensors, which is important to not only keep the battery from over temp but is used to calculate the specific voltage needed referenced to temperature.

they have programmable settings for all types of battery technologies,, at least the major ones.

voltage set points are programmable, as well as time functions etc.

and a bunch of other functions, all programmable

the retail cost for a 24 volt version is about 300 bucks, or so

ebay for around 200 maybe less

again i am not sure if they are available in 48 volt, but a call to the oem

tech dept might allow one to be used on a 48 volt system.

i know how it could be done, but will refrain from making a recommendation

check out the balmar products, and read their FAQ

Xantrex also markets one, that looks lot like the balmar, might even be

a rebadged balmar i dunno.

ample power makes one as well

there are a couple of manufactures in england as well.

the boating folks use these controllers alot, so check out the boating forums

if you have other questions specific to your application.

in reality, boating and offgrid have a lot in common in my opinion. the boating

guys have been at the offgrid thing a very long time, so i figure why not learn from their experience. why reinvent the wheel?

you asked why not run the alternator without the controller?

if you build such a system, you will either end  up with

1. a battery charging babysitters nightmare, or
   
2. an instrument of torture for the batteries, or
   
3. a murder weapon of the batteries, or worse yet
   
4. a combination of the three
   
   

without some form of control with temp sensors you risk

overheating the alternator, burnout resulting with increased costs involved

without some form of control with temp senseing of the batteries

you risk damage to the batteries from overheating, under or overcharging

unless you are very attentive and careful.

one can certainly charge the batteries effectively and safely if he is careful

and monitors the process, that requires babysitting them over the complete charge

regime.

or he can use a controller, set it up correctly and not have to stand watch

i guess it all depends on how much your time is worth.

basically if i had a large set of expensive batteries that i were to depend upon

in an offgrid installation, i would step up to the use of a controller.

even if i had lots of time on my hands.. sooner or later i would be absent, sick or just make a stupid miscalculation and end up costing me more than the cost of a controller in battery or alternator damage.

let alone fuel consumption, which is also a significant factor.

the controllers are designed to charge the batteries in the most efficient manner possible, that means lower run times and less fuel burned.

then there is system life span, lower run times usually result in longer life of the components, engine and alternator.

lots of factors to consider

bob g

bob g

[Usman]

Great explanation, thank Bob g, I just hope that I can find a 48V version.

[ONEmule]

 Idling any diesel is not recommended as it will cause cylinder walls to score. you must check with manufactor to see what is best RPM range for diesel. but do not idle