Battery sums please

simonfraser

simonfraser

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one 12V red top battery and one 12V Trojan 105 Ah, both fully charged, both floating at 13.5

KAD connected to the red top
VSR between the two batteries
Nothing else connected

(set up my choice - dont get picky pls)

Start the KAD, the voltage of the red top drops to 10.5V during the 8 seconds it takes to start, the VSR supplies energy from the Trojan to the red top due to the voltage difference
Just for ease of calculation, assume the VSR has no resistance and unlimited current carrying ability

How much extra current will the Trojan supply to the red top / KAD ?

How much extra current will two Trojans supply to one red top / KAD ?
 
simonfraser said:
one 12V red top battery and one 12V Trojan 105 Ah, both fully charged, both floating at 13.5

KAD connected to the red top
VSR between the two batteries
Nothing else connected

(set up my choice - dont get picky pls)

Start the KAD, the voltage of the red top drops to 10.5V during the 8 seconds it takes to start, the VSR supplies energy from the Trojan to the red top due to the voltage difference
Just for ease of calculation, assume the VSR has no resistance and unlimited current carrying ability

How much extra current will the Trojan supply to the red top / KAD ?

How much extra current will two Trojans supply to one red top / KAD ?
Click to expand...

I'll be interested to see replies from others. I don't know what a KAD is, how much power it consumes, length of various cables involved, cable diameters, number of connections, condition of batteries etc.

There's probably a 3V difference between the 2 batteries so I suppose you are asking how much current will a particular model of Trojan battery deliver when fully charged and delta 3V from 13.5V starting point. This would make more sense than calculating a current using zero resistance you quote for VSR.
 
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Yep:

‘There's probably a 3V difference between the 2 batteries so I suppose you are asking how much current will a particular model of Trojan battery deliver when fully charged and delta 3V from 13.5V starting point’

The Trojan is model 27TMX

The starter on the KAD is quoted at 12V 1.7kW

Ball park figure ?
 
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I don't think that a realistic calculation is really possible. The resistances in the circuit will have to be accurately known, ie. both battery internal resistances, connection resistances, let alone the VSR, which you suggest is zero.
These figures will be milliohms and would be difficult to determine.
All IMO.
 
Don't be fooled into thinking the peak current is 1700w/12V=142A .
Until the starter starts turning, the surge current is only limited by resistance and inductance.
In the first milliseconds, each battery might be putting out hundreds of amps.
If you add a third battery, that too might peak at hundreds of amps.
After say 2 seconds, when the engine is turning at a steady-ish speed, the current will be much less, and mostly dictated by the actual work the starter is doing.
 
Mistroma said:
I'll be interested to see replies from others. I don't know what a KAD is, how much power it consumes, length of various cables involved, cable diameters, number of connections, condition of batteries etc.

There's probably a 3V difference between the 2 batteries so I suppose you are asking how much current will a particular model of Trojan battery deliver when fully charged and delta 3V from 13.5V starting point. This would make more sense than calculating a current using zero resistance you quote for VSR.
Click to expand...

Engine! ............. 170hp 4 cyl or 260hp 6 cyl turbocharged Volvo Penta power boat diesel engine.
 
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simonfraser said:
one 12V red top battery and one 12V Trojan 105 Ah, both fully charged, both floating at 13.5

KAD connected to the red top
VSR between the two batteries
Nothing else connected

(set up my choice - dont get picky pls)

Start the KAD, the voltage of the red top drops to 10.5V during the 8 seconds it takes to start, the VSR supplies energy from the Trojan to the red top due to the voltage difference
Just for ease of calculation, assume the VSR has no resistance and unlimited current carrying ability

How much extra current will the Trojan supply to the red top / KAD ?

How much extra current will two Trojans supply to one red top / KAD ?
Click to expand...

The VSR should open when the volts fall below the spec'd opening figure but close again as the engine alternator recharges the engine start battery and the volts rise to the spec'd closing voltage.

Exact behavior will depend upon the VSR fitted, which you dont tell us.
 
VicS said:
The VSR should open when the volts fall below the spec'd opening figure but close again as the engine alternator recharges the engine start battery and the volts rise to the spec'd closing voltage.

Exact behavior will depend upon the VSR fitted, which you dont tell us.
Click to expand...

Most VSRs these days are specifically desinged not to open during the starting surges, as it tends to result in a very short life for the contacts.
 
Can't see a need for "sums", including fictional assumptions, just measure the current passing through the VSR.

If the mains charger has been off for a while and the battery voltage has fallen below 13v the VSR will be open anyway. If the voltage at the Trojans falls below 12v the VSR will open in 1 second. If the VSR disconnects it will not close again until the alternator has been charging for 5 mins (in the OPs case).
 
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As said above find someone with a DC clamp meter & measure it is probably the only accurate way - so many variables.

That said, this might be of interest -
https://pssurvival.com/PS/Batteries/Lead_Acid/Lead-Acid_Battery_State_Of_Charge_Vs_Voltage_1993.pdf

12V_Lead_Acid_Discharge_Curves.jpg
 
PaulRainbow said:
Can't see a need for "sums", including fictional assumptions, just measure the current passing through the VSR.

If the mains charger has been off for a while and the battery voltage has fallen below 13v the VSR will be open anyway. If the voltage at the Trojans falls below 12v the VSR will close in 1 second. If the VSR disconnects it will not close again until the alternator has been charging for 5 mins (in the OPs case).
Click to expand...

Why does the VSR want to close if the Trojan drops below 12 v ? if it has closed at below 12 volt what is the disconnect point ?

Brian
 
VicS said:
Engine! ............. 200(ish ) hp turbocharged Volvo Penta power boat diesel engine.
Click to expand...

Straight 6, 3.6l turbo charged and supercharged, 260hp. Typically fitted in pairs to circa 35-40ft flybridge cruisers. Big lumps to be hooking up to Red Tops, IMO.
 
I don't know the batteries mentioned however one might expect as an answer to the hypothetical question that both batteries would deliver a similar current if in parallel on start up. The smaller engine start battery being designed for high cranking amps (for it's size and cost) versus the deep cycle battery which while not designed to deliver high current will do so simply because it is big.
The engine start battery should be sized to start what is obviously a big engine(s). 24v system would be better but 12v is oK. You should always start just on the engine start battery. This way if and when it starts to fail to do the job you will know immediately and will be able to connect the big deep cycle battery to assist and get you out of trouble. If you as many do habitually start on both batteries you will not know you have a problem until both batteries together will not start the engine. So no plan B. Some VSR may be designed to take starter loads with a separate control to do so but generally it would be safer to have a dedicated emergency (jump start) switch to take the high current. olewill
 
K, so the larger trojan(s) will have a higher voltage than the red top when the starter is turned.
This activates the VSR and there will be a significant current flow to the red top, agreed ?

Yes, i can and do use the x over switch to combine the red top and the 2 Trojans to start the KAD 44.
On a cold day the existing 35A fuse in the VSR blow when starting the KAD.
I am increasing the fuse & wire for the VSR which may avoid the use of the x over switch.

This may be due to the one red top being too small for a big V8 diesel lump, even though it starts @ 0C no prob.
If you go to the red top battery selector site, and select a big V12 from a truck / merc they do not advise a red top.

So, just hypothetically, if i had two red tops, the current flow via the VSR would be less, i guess ?
Not sure how accurate my clamp meter would be with the initial Amps used, and my arms are not long enough to start and see the display anyway :)
 
simonfraser said:
K, so the larger trojan(s) will have a higher voltage than the red top when the starter is turned.
This activates the VSR and there will be a significant current flow to the red top, agreed ?

Yes, i can and do use the x over switch to combine the red top and the 2 Trojans to start the KAD 44.
On a cold day the existing 35A fuse in the VSR blow when starting the KAD.
I am increasing the fuse & wire for the VSR which may avoid the use of the x over switch.

This may be due to the one red top being too small for a big V8 diesel lump, even though it starts @ 0C no prob.
If you go to the red top battery selector site, and select a big V12 from a truck / merc they do not advise a red top.

So, just hypothetically, if i had two red tops, the current flow via the VSR would be less, i guess ?
Not sure how accurate my clamp meter would be with the initial Amps used, and my arms are not long enough to start and see the display anyway :)
Click to expand...

No necessarily.

Normally a VSR only closes when it senses a voltage above its specified "closing voltage" Typically that is around 13.4/13.6 volts. They dont operate by sensing a difference in voltage between the the batteries .......... Although some ( eg the Cyrix which PR recommends) do have a selectable a facility to operate in " start assist mode"

What VSR do you have

( PS we do know that you only have a single KAD44 EDC engine! )
 
simonfraser said:
K, so the larger trojan(s) will have a higher voltage than the red top when the starter is turned.
This activates the VSR and there will be a significant current flow to the red top, agreed ?
....
Click to expand...
Your first post said the two batteries were floating at 13.5V.
It's this voltage which causes the start point to be that the VSR is closed.

If you start with the batteries 'resting' at say 12.7V, the VSR would be open.

You stand a good chace of getting bad advice if you give people bad information to start with.
The obvious thing to do is to turn off the VSR before attempting to start the engine.

You should IMHO, get a starter battery fit for the purpose of starting the engine.
Or abandon the pretence of an engine battery and just wire the starter to the house batteries.
You appear to be just complicating a state of denial that your engine battery doesn't do the job.
Red tops were very fashionable in the LandRover community for a while, but many weekend Landyboys seemed to get problems with them quite quickly.
 
simonfraser said:
yes its a cyrix, the start assist is a manual extra switch which is not being used.
one KAD 44 only.
Click to expand...

So if you are not using the start assist facility it wont close in the way you seem to think due to the starter battery voltage falling below the house battery voltage. It will only close after the engine has started and the starter battery volts have risen above 13.

i
 
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