Charging a third battery in the boot via external Anderson plug

Submitted: Thursday, Oct 31, 2013 at 20:07
ThreadID: 104947 Views:4591 Replies:4 FollowUps:9
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I've just had a dual battery system installed. Second battery under the bonnet. Two runs of 6b&s cable to the rear. One terminated with a pair of internal cigarette sockets and the other with an Anderson plug on the towbar. I want to charge a third battery in the boot of the Prado from this external Anderson plug. I've seen these cables on Ebay rated to 50 amps with an Anderson plug at one end and a pair of lugs at the other end. Is this safe and/or practical considering I'd have to slam the boot shut on this cable? Are there other alternatives like a flat cable?
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Reply By: Ross M - Thursday, Oct 31, 2013 at 20:28

Thursday, Oct 31, 2013 at 20:28
Many vehicles have rubber plugs or metal blanking plate in the floor and your Anderson cable or a plugged in extension maybe able to be fed up through one of those, if present, and feed a third battery.

Ensure it is protected with many layers of cushioning from edges of metal bodywork and securely cable tied with many ties to prevent movement of the cable.
Large convoluted split plastic like manufacturers use along with grommets etc, and fuses at both ends of the cable is good insurance.

Ross M
AnswerID: 520694

Reply By: Echucan Bob - Thursday, Oct 31, 2013 at 22:54

Thursday, Oct 31, 2013 at 22:54
Even with 4 mm wire you may get a fair voltage drop and the back battery may not ever fully charge. I agree with Ross, fuse each end, protect the wire throughout its length, and run it through a grommet where it enters the bulkhead. To avoid voltage drop you can use heavier wire, or employ a DC to DC converter by the likes of Redarc or C Tek.


AnswerID: 520701

Follow Up By: Ross M - Friday, Nov 01, 2013 at 08:21

Friday, Nov 01, 2013 at 08:21
You only get a significant voltage drop when the demanded amps are high. As it gets towards the end of charge the difference of voltage will be less and so are the amps, therefore the battery will get to very close to fully charged. It is only within the vehicle and not to the rear of a camper trailer.

There is no substitute for heavy wire though.
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Follow Up By: Brian 01 - Friday, Nov 01, 2013 at 09:05

Friday, Nov 01, 2013 at 09:05
1. A DCDC charger is no substitute for adequate wire size.
Watts in equals watts out - there's no free lunch.
Whilst it will boost the voltage at the other end, it will require more current from the input to do so, and this will be at the expense of even greater voltage drop, which will then require ever more current, and so on.
The eventual result, if the wire is too small, is a piddling charge to the battery, or even that the DCDC device ceases to proceed.

2. Whilst it's true that the voltage drop will decrease with load amps, the problem is that you need lots of amps in the early charge stage to get the battery to that low load point.
The reality is that you may just not drive far enough to achieve that point of charge, and the lower the charge voltage or current, the longer that drive will need to be.
FollowupID: 801185

Follow Up By: Ross M - Friday, Nov 01, 2013 at 10:11

Friday, Nov 01, 2013 at 10:11
I agree, so with heavy cable which allows the likely amp flow which can be delivered from the alternator, it will charge the battery because there isn't a lot of voltage drop at all.
The DC from the alternator charges the battery with DC ie, natural DC/DC charging I reckon.

It all works like that in my setup.
FollowupID: 801190

Follow Up By: Brian 01 - Friday, Nov 01, 2013 at 11:20

Friday, Nov 01, 2013 at 11:20
Except that the alternator regulator is looking at the terminal voltage of the crank battery, which will rise more rapidly (due to voltage drop no matter how small) than a more remote battery and will taper its output current to suit that rising crank battery voltage.
This means that the remote battery does not get as large a charge current as it might otherwise receive.
A DCDC charger, on the other hand, is seen by the regulator as a load, just like the headlights etc. and so it will boost its output to accommodate that load.
The difference is caused by the relative voltage depression that is produced at the regulator sense point.
The remote battery will be at most a couple of volts lower than the crank battery, this will only slowly depress the crank battery terminal voltage as it sends charge to the remote battery (again limited by wire capacity).
The alternator regulator will quickly replace that charge and will frequently ramp down as the crank regains its set voltage.
The regulator is a constant voltage device, its job is to simply keep the system voltage at a set level (within its capability) irrespective of the load.
The DCDC charger in boost mode is a constant current device, and so it sends a constant set current to the remote battery irrespective of the supply voltage, again within the capability of the device to do so.
This constant current draw causes the alternator regulator sense voltage to become, and remain, depressed, so it increases and maintains its output to compensate.
If you have a close look at an alternator charging a second battery you will see that it may put out a nice high charge current for a maximum of a few minutes, but will very quickly fall to a fraction of its starting point, this is due to the voltage sensing of the regulator, and is the reason why a DCDC charger will always do a better job.
Sorry it's so long winded, I get carried away BIT.
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Follow Up By: olcoolone - Friday, Nov 01, 2013 at 13:13

Friday, Nov 01, 2013 at 13:13
Alternators were ever only designed to charge a starting battery to an acceptable level (not a full charge level) and are temperature compensated as most starter batteries are in the engine bay..... hotter the engine bay - hotter the battery - low the charge rate.

A DC-DC charger will charge an auxiliary battery to near 100% SOC, and will provide the correct voltage and limit inrush current to a more acceptable safer level plus lowering the load off the alternator..... alternators all have a duty cycle, so if it's a 70amp alternator it doesn't mean it can deliver 70amps constant...... good way of destroying alternators.

All the vehicles we fit DC-DC chargers to old and new; we see a good improvement on the auxiliary battery charge rate of 1-1.5volts over the starter battery.

No matter what; there is no substitute for battery cable size even with a DC-DC charger.

Brian you will come across many who don't think DC-DC charging is needed and is a gimmick. if it was fluid; voltage would be pressure and current; the quantity. Higher pressure over comes resistance and the quantity can flow.
FollowupID: 801199

Reply By: Member - Sn00py2 (NSW) - Friday, Nov 01, 2013 at 22:03

Friday, Nov 01, 2013 at 22:03
Hi Garbage,

You could always install an Anderson plug in the boot of the car like I did. I plug a DC/DC Ctek charger into this Anderson plug and use it to charge my fridge battery.

AnswerID: 520741

Reply By: The Bantam - Friday, Nov 01, 2013 at 23:37

Friday, Nov 01, 2013 at 23:37
On a completely differet line.

This third its inside the vehicle...hmmm
How is it restrained...hmmm

as far as safety and good practice is concernd batteries should always be charged in places that are properly ventilated to the outside air.....there is no such thing as a totallt sealed battery..all lead acid batteries can and will vent explosive gasses and acid mist if over charged.

Batteries are pretty heavy is this 20 pluss KG lump of lead restrained in the vehicle....yeh its inside the passenger compartment.

I know a bloke who to this day has back problems from being hit in the back by a battery in his rally car.....that is hit in the back of the seat, and this battery was secured by metal rods and plates welded under the floor.
If a 20 pluss Kg lump of lead gets loose inside the cabin in a rollover it can become a real good head smasher....add the complication of acid from the smashed battery..and it looks less and less a good idea.

on the matter of voltage drop
you all would be surprised how light a cable can be used to charge a battery.
The light size of the cable may slow the charge rate, but it will not prevent the battery from being fully charged given time.
As the battery gains charge the charge will taper and the voltage drop will reduce.

Besides...this socket the original poster talks about is #6 B&S...thats bigger than 12mm2...of course you will charge a battery effectivly thru that

Hell most jumper leads are not that heavy.

AnswerID: 520743

Follow Up By: Echucan Bob - Saturday, Nov 02, 2013 at 12:12

Saturday, Nov 02, 2013 at 12:12
Bantam, I think #6 is 4.1mm diam.
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Follow Up By: The Bantam - Saturday, Nov 02, 2013 at 22:05

Saturday, Nov 02, 2013 at 22:05
Number 6 (or 6 guage) Battery and Start is about 13.5mm2
6mm (nominal) automotive is about 4.5mm2

that is according to the automotive section of my Tycab catalogue.

FollowupID: 801288

Follow Up By: olcoolone - Saturday, Nov 02, 2013 at 23:29

Saturday, Nov 02, 2013 at 23:29
And we should not get the battery and starter thing confused with B&S that cable is measured in.......B&S does not stand for battery and starter but Brown and Sharpe who set the standards for AWG (American wire gauge) specifications early last century.
FollowupID: 801292

Follow Up By: The Bantam - Sunday, Nov 03, 2013 at 22:05

Sunday, Nov 03, 2013 at 22:05
Yes there is some confusion.
In general automotive cables not for battery and starter use are simply refeered in imperial terms to as "such" guage.....with the inherant confusion between american and british guages....which more or less agree to a point but then diverge.

correctly or not.
cables for battery and start use are refeered to as such guage B&S.
And the catalogues have the words "Battery and Start" at the top of a different table of cables than for other use.

Battery and start cables in general do not come in much choice of colour and may be coarser strabded than the other cables.

Then we get the whole metricated guaged cables....cables made from metric strandings that approximate guage sizes but are neither, clean even guage size nor, clean even metricated size.

OH and the stupidity that is refeering to a cable by its insulation diameter rather than its metalic cross sectional insulation diameter that due to improvements in plasitcs is nothing like accurate.

so we have a 6mm ( insulation diameter ) cable that is in fact 4.15mm nominal OD, is 4.59mm2 metalic crossectional area and is near, but not right on 10 1/2 AWG

This is all an infernal irritation.

For goodness sakes people discuss cable, like the real electricians and the engineers do ..........only in metalic crosssectional area and in have to convert to crosssectional area to do any calculations anyway.

FollowupID: 801367

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