New article on Battery Power

Submitted: Friday, Feb 24, 2006 at 21:26
ThreadID: 31120 Views:2724 Replies:6 FollowUps:5
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The new article on Battery Power is a great start to explain the basics of planning your DC power, but hopefully some improvements will be made so that it doesn’t perpetuate some of the common misunderstandings about DC power.

“the fridge is on, the camera and laptop chargers run off a 600 watt inverter for 3 hours and the 12 volt lead light is used for 5 hours. Total amp draw is 60 amps.”
- no, the total capacity taken from the battery when these loads are used for this time is 60 amphours. Battery capacity is correctly described in amphours in the earlier paragraph in this article. If you subtract 60 amphours from the battery’s 90 amphour capacity, you know you have 30 amphours left in the battery. Amps is used only to indicate the instantaneous load and doesn’t help you understand how much capacity it takes from the battery. If a starter draws 300 amps for 6 seconds how much has it discharged the battery ? You need to calculate amphours - 6 seconds is 1/600th of an hour - so it has taken 300 x 1/600 = 1/2 amphour.

“------Device------------------------------- Usage (amps per hour)”
“ 12 volt fridge (compressor type)----------2.5-3.0 “
- A fridge draws 5 amps when the compressor is running and 0 amps when it is not. So to give an indication of how much power it draws on average, you could call it 2.5 amps average or 2.5 amphours per hour.

“Wet Cell / Marine Battery / Deep Cycle and Gel Batteries / AGM”
- this suggests you need to choose between Wet Cell OR Marine OR Deep Cycle OR Gel OR AGM.
- the electrolyte storage can be wet OR gel OR flooded and the plate construction can be Starting OR Marine OR Deep Cycle. So if you decide to buy a Wetcell, you can still choose between Starting OR Marine OR Deep Cycle plate construction. AGMs are available as Starting OR Deep Cycle.

“The minimum cable thickness that should be used is 100 amp 8 B&S cable (B&S stands for battery and starting cable).”
- no, 8B&S wire is rated closer to 56 amps. Keep in mind that the current rating for a wire only tells you much current you can put through it - without the insulation melting. If you are concerned about voltage drop, you need to consider the length of wire, as well as its size.
- no, B&S stands for Brown & Sharp, the name before it became the American standard and was then called AWG. 30B&S wire is .25mm in diameter - only for starting very small motors.


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Reply By: Eric Experience. - Friday, Feb 24, 2006 at 21:59

Friday, Feb 24, 2006 at 21:59
The article certainly leaves a lot to be desired, a lot of vehicles catch fire from bad wiring like is described in the article. It is easy to see the products that are sold by the author. Eric.
AnswerID: 156931

Reply By: roger baker - Friday, Feb 24, 2006 at 22:34

Friday, Feb 24, 2006 at 22:34
I agree with Mike with the following addition.
Fuses are inserted in circuits to protect both the the wiring and the appliance.
If you dont have a fuse and you do get a short circuit in the system you WILL have a very expensive fire, probably at the most inconvenient location.
Seen it happen and the vehicle was a write-off

AnswerID: 156935

Follow Up By: Mike DiD - Friday, Feb 24, 2006 at 22:45

Friday, Feb 24, 2006 at 22:45
. . and if the Insurance Companies finds that the fire was caused by an unsafe modification to the vehicle, they won't pay a cent.

FollowupID: 411116

Reply By: Wok - Saturday, Feb 25, 2006 at 07:58

Saturday, Feb 25, 2006 at 07:58
The article would benefit with the inclusion of diagrams,tables etc; this would clarify the interpretation of the written word. [A picture is worth...blah...blah]

On the positive side, its a starting point, hopefully the author will be receptive to "changes" based on EO's collective expertise/opinions.

Just my dinars worth :}


AnswerID: 156956

Reply By: arthurking83 - Saturday, Feb 25, 2006 at 16:48

Saturday, Feb 25, 2006 at 16:48
Can someone please explain to me what this means...

"If you look at the graph below, you will see the charge zone of a 85 amp alternator is 2000 to 4000 rpm, which means a very fast idle speed or a slow drive for the required charging time."

I'm assuming the "charge zone" of the alternator (2000-4000rpm) is the actual atlernator speed not the engine speed????
(that's why they are geared down so much!!)

So what this guy is saying that in order for the alternator to charge the battery properly, it must be kept within 2000-4000rpm?

The info provided is very confusing and it seems wrong! Unless I have mis-read his explanation!

My understanding of alternator charging is, that you need to keep the engine just above idle, in order for the alternator to provide full charging current to the battery......therefore any engine speed [*above*] (eg.) 1000rpm will enable the alternator to provide full charging current (and only limited by the regulator)?

AnswerID: 157020

Reply By: Member - Collyn R (WA) - Saturday, Feb 25, 2006 at 20:56

Saturday, Feb 25, 2006 at 20:56

Alternator specifications are almost always shown in alternator rpm. The following is Bosch data but is typical of most.

A typical alternator is 'geared' such that constant long-term electrical needs are fulfilled at 1500-1800 alternator rpm - as you say, around engine idling speed.

The rated maximum current is typically produced around 6000 alternator rpm.

Maximum safe speed is very much higher(15,000 rpm) but is rarely if ever exploited.

The starter battery is initially charged (for the first few seconds at 20-50 amps but this falls to about half that within a typical 30 seconds and to only an amp or two about a minute after. That battery normally regains its charge within two to three mnuts of starting.

Paralleled deep cycle batteries are likely to charge at 15-20 amps if fairly low in charge - but as the charging process is constant voltage, charge current falls off as current rises. It will be thus down to a few amps once the batteries are much past 60% or so charge. It tapers to a trickle shortly after.

AGM batteries may draw more.
Collyn Rivers
AnswerID: 157053

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