B2B fitting/split charge

[davenewellhome]

"In terms of manufacturing costs it must only be pennies per van so why do they do it?"

 

Quite simply because it doesn't make enough of a difference to the way the leisure battery(ies) charge from the alternator. What actually happens is the alternator gives a highish current to start with and a large part of this goes into the engine battery to replace the energy used to start the motor. Then, very soon afterwards the charging current falls off to a low level as the battery voltage comes up quickly (usually referred to as "surface charge"). Adding more batteries via heavy cable will extend the period of higher current charging but only by a small amount.

 

The Stirling unit by comparison fools the alternator into producing up to 50 Amps continuously with no fall off. It does this by loading the engine battery to drag its voltage down thus ensuring that the alternator will give a continuous high charge current. Periodically during this high current charging it shuts down the charge to the leisure battery and removes the load from the engine battery. It then checks both batteries charge state and if it finds the leisure battery is still low resumes its charging regime. If it finds the engine battery wanting it allows it to take the charge.

 

This is my understanding of the system anyway.

 

D.

[Syd]

Hi Clive.

A begging letter

In your reply to Rich you give out some very useful information but it relates to motorhomes.

I have a caravan so how do I get this same/similar setup to work on my caravan.

 

Would you recomend cables to the rear of the towing vehicle terminating in some type of plug and then cables into the caravan for the leisure batteries.

Or would you recomend something else, or not attempting anything like this in the first place

[Clive]

If a leisure battery has been used for wild camping for a couple of days such that it is sufficiently discharged then it will take a substancial charge for some time directly from the alternator. Yes the alternator voltage is limited to a prety accurate 14 volts and this is the critical gassing voltage for a 6 cell lead acid battery. Conventional split charging is better with low resistance (fat) cables without doubt and will enable a leasure battery (and the starter battery) to get to around 80% charged in a reasonable time. But super fast charging does have its drawbacks. Heat, reduced cycle life of the battery, potential buckling plates and a need for more regular topping up of the electrolyte.

In commercial applications where fast charging is required yes temperature sensors are fitted to monitor the battery temperature, however the batteries also have plumbing built into the cells so that air can be bubbled through the batteries to both stir the electrolyte and also aid cooling by distributing the heated electrolyte. On top of this "Autofill" systems are used for quick one shot topping up the electrolyte.

If this fast charging system were that good the charging algorythm could be incorporated into the alternator regulator for zero cost. The reason its not is because of the associated drawbacks and risks associated with it in an automotive application.

If you need the ultimate amount of energy from your leisure battery then you should change it for a bigger one where 80% charge in a reasonable time will be sufficient.

Taking the battery voltage above 14 volts during a SLOW charge is far kinder to a leisure battery which after all is designed (hopefully) as a deep cycle battery and not a starter battery which is more able to accept high discharge and charge currents.

Bunging 50 amps into a 100 ampere hour 6 cell leisure battery with a terminal voltage above 14 will not do it a lot of good. Potentially charging it in 2 hours is well out of its cumfort zone. Even two batteries in 4 hours is pushing it very hard. I know what the sales literature claims but I remain unconvinced.

C.

 

 

 

 

 

[Clive]

Syd,

Caravans have the disadvantage of that piddling 7S socket which is a little small and consequently has a higher contact resistance than would be prefered. Then there is the limit of the cable size that the 7S socket can take. Other than that everything else in the arguement remains the same. Don,t forget the negative return from the van to the car chassis either.

 

Small versions of the Stirling system can be used to augment the charging voltage but I am very wary of high charge currents at voltages above 14 at the battery terminals.

 

 

[Syd]

Clive - 2008-02-06 9:26 PM

 

Syd,

Caravans have the disadvantage of that piddling 7S socket which is a little small and consequently has a higher contact resistance than would be prefered. Then there is the limit of the cable size that the 7S socket can take. Other than that everything else in the arguement remains the same. Don,t forget the negative return from the van to the car chassis either.

 

Small versions of the Stirling system can be used to augment the charging voltage but I am very wary of high charge currents at voltages above 14 at the battery terminals.

 

 

Wow, That was quick Clive, thanks a load

So I run a heavy cable as mentioned in your reply from the + connector on the alternator, along with another same size cable for earth, to a robust connector at the rear of the vehicle.

I then run two same size cables from the leisues batteries to the heavy connector on the vehicle.

Im talking about a total run length for the cables of about 22ft +,

will it be worth it for the voltage drop on that length??

[Clive]

Every little (bit of saved voltage drop) helps, don,t forget the big fuses.

 

But you could always flog the caravan and get a motorhome!

 

Good luck

 

Clive

 

[Syd]

Thanks so much Clive, you are a star

 

Couldn't swap the caravan for a motorhome even if I wanted to.

Because of Lee,s problems we need lots of room.

Even had to widen the door to get his lecy chair in.

I absolutely love this caravan even though it is a 1999 model

 

[Clive]

Now, there is another aspect we havent covered, charging up the leccy chair in a timely manner.

 

Good luck though

 

C.

 

[Syd]

Clive - 2008-02-06 10:55 PM

 

Now, there is another aspect we havent covered, charging up the leccy chair in a timely manner.

 

Good luck though

 

C.

 

Well the lecy chair does last for about two/three weeks as we use the manual chair mostly, simply because you cannot get to places as easilly with the lecy chair.

But this is an interesting one that I have not considered as yet.

 

Come on then Clive how do we best do it??

At home when it is pretty well run down it takes all night to charge it back to full

[Guest Tracker]

Clive,

 

I too would be uneasy with a 50 amp charge belting into my leisure batteries for the reasons that you give and I would much prefer to get the existing 30 amp relays to handle maybe 20 amps instead of the current 10 amps current!

 

That should top my batteries in a more sensible number of miles on the road.

 

So maybe if I keep the existing relays and just uprate the cables to carry say 25 amps from the alternator to the leisure batteries, and the fuses to say 30 amps, together with crimped or soldered terminals rather than push on Lucar style joints?

 

Would that achieve anything please?

[Syd]

Tracker

 

Surely if you have two leisure batteries wouldn't the 50amp charge then be the equivalent of 25amps each??

Thus gettin close enough to your prefered 20amps

[Guest Tracker]

Syd,

 

Potentially a good point, but I don't think so because, I think, the current coming down the cable hits both batteries simultaneously because they are wired in parallel - but Clive or Dave will tell us for sure!

 

My trouble is that I know just about enough to fix modern things sometimes but not enough to alter them with complete confidence and I have seen just how a big battery can go with a big bang if you get it totally wrong!

 

It was so easy with dynamos and regulators - they broke down so often that fixing them became second nature! Progress eh!

[Clive]

Yes, 50 amps charge current equally shared between two similar batteries will be 25 amps each. The thicker and shorter the cables the better and don,t forget to make the pick up point the B+ terminal on the alternator.

Re charging a mobility scooter battery I did once give some advice for someone who had one and it did use a 12 volt lead acid battery. My simple suggestion was again to put in a second split charge relay to charge the scooter battery. It would only take a proportionate amount of the charging current.

[Syd]

Hi Clive

Thanks a lot for the info.

Just purchased Lee a new lecy chair to be delivered when they get his seating and back cusions.

These things last so long that they are almost antique and still running perfectly. We wanted him to look modern

Suspicious that the batteries may not be lead acid ones though and that may change things

[Guest Tracker]

Thanks Clive - I continue to learn!

 

As a further display of the gaps in my knowledge I wonder what determines the current flow.

 

Given that the alternator is belting out a charge of up to 80 amps, is it the limitations of the connections or the limitations of the cable - or have I missed something?

 

When a high current tries to flow along a restricted path the end result is usually heat and, in extreme circumstances, fire starting at the point of highest resistance - unless a fuse blows first to protect the set up.

 

So with a cable run direct from the alternator output is it OK to use cables and fuses with a lower capacity than the ultimate alternator output?

 

Needless to say I am not changing anything that might endanger us or the van and I would rather put up with the measly 10 amp charge rate than risk a fire.

 

Pardon my ignorance please Clive!

[Clive]

Good questions Syd,

 

Current flow is determined by Ohms Law. That is that the current flowing in a circuit is directly proportional to the applied voltage and inversely proportional to the circuit resistance. In formulae terms I(amps) = E(volts) devided by R (resistance in ohms)

 

In our example we have the the alternator that will within the limitations of its maximum current rating try and maintain 14 volts at its terminals.

We also have the battery which can have a terminal voltage nominally of 12 volts but frequently also at 14 volts when charged. If pancake flat it can be below 12 volts but it will not remain there for long once on charge.

 

The voltage in our ohms law calculation is the difference between the alternator voltage and the battery voltage. So typically the voltage for the formulae will be about 2 volts initially and will fall to zero volts when the battery is charged. With such a small voltage you can now see why circuit resistance is important.

 

The resistance will be that of the cables + the relay contact resistance + the fuse and holder + the internal resistance of the battery + the negative return path often via the chassis.

 

Small batteries typically have a higher internal resistance than larger batteries. Also starter batteries typically have a lower internal resistance than leisure/traction/deep cycle batteries but are not designed for deep cycling.

 

Does that help?

 

C.

 

 

[Clive]

That should have started "Good Questions Tracker" !!!

 

I will wake up soon.

 

 

[Guest Tracker]

Clive - 2008-02-08 4:46 PM

 

That should have started "Good Questions Tracker" !!!

 

I will wake up soon.

 

 

That's OK Clive - I too am familiar with senior moments!

 

Thanks for the much needed reminder of the basics - it's a blooming long time since my 'O' level physics!

 

I think I deduce that the limiting factor in the amount of current reaching the battery from the alternator is indeed the resistance of the length and the diameter of the cable plus its connections and fuses - I think.

 

So do I also deduce that the reason the whole lot does not overheat is because it simply cannot pass enough current to do so unless the voltage difference rises?

 

Forgive me for being a thicko - but if the voltage differential is nil can I presume that no current will flow and that is what stops a battery from boiling itself dry?

 

There must therefore be a formula for working out the optimum size of cable to produce the maximum current flow at any given voltage differential - in this case maybe a low of 10v and a high of 14 v = 4 volts differential?

 

I am sure that many others on here who maybe as dumb as me but afraid to say so will be equally enthralled and captivated by Professor Clive's explanations - and if not they bloody should be because it is a fascinating subject and I should have paid more attention at school!

 

Just one more - which is better at conducting current - one very thick single strand (not very flexible), a bunch of fine strands (for maximum flexibility) with the same total copper cross section , or a group of strands in between (less flexible but each strand carrying more current)?

 

Thanks Clive - much appreciated!

[Syd]

Damn it Clive I was just going to post that

 

Thanks a bunch

[Clive]

The limiting resistance is the total resistance of all the circuit added together. The internal resistance of the battery not being insignificant.

Solid strand or multi stranded?. At DC it makes no electrical difference, however fine stranded welding cable is best for mobile applications because its better mechanically iprotected and more durable than mains tails type of cable.

At high speed AC frequencies fine stranded cable is best because "skin effect" causes the current to mostly flow on the surface of the conductor.

When the battery is charged only sufficient current will flow to keep up with the batteries self leakage and maintain 14 volts. Of course some intelligent chargers allow the voltage to go above 14 for a short time but only when the current is low, plus many also monitor the battery temperature. But not at high currents.