[SteveSS]

I just noticed that Harbor Freight has a new lithium jumper on sale... gets great reviews. I can't figure out how it (and yours) works so well... seems like it just wouldn't be strong enough.

Good segway for another thread ... So, how many amps does a starter draw? I have a BlueSea ML-ACR (rated to 500A) with a jump-start feature like a BatteryDoctor. BlueSea says no fuses if it's ever going to be used that way. For charging-only applications, they spec a fuse at both battery ends (starter and house bank). Oddly, BD only specs a fuse at the house bank end (150A). That seems kinda low given that Google says starters draw closer to 250A.

I'm not comfortable with going fuseless just for the rare jump start. What size fuses (both ends) would support jump starting and still adequately protect the normal charging circuit (i.e., big but not too big)?

 

[RDinNHandAZ]

500 amps should do it! Perhaps it would be better to open the connection to the starter battery and give them a few minutes to drain some juice from the house battery to the starter battery then start the van with the starter battery? I too cary one of those Lithium compact jump starters that is supposed to start a 5 liter diesel IIRC. It says it can provide 600 amps! It seems to be 18 Amp Hours at 12 volts. If one believes the hype. I’d try that first, the process above second, Jumper cables from my house battery to the front jump spots third and the system you are asking about last.

 

[SteveSS]

Thanks, RD. I just want to make sure a really big fuse won't somehow compromise the charging aspect. You never know about those tricky electrons. You also make a good point about draining some juice for a few minutes. I think I'll install readily available 250A mega-fuses at both ends to begin with, and carry spares.

 

[RDinNHandAZ]

Actually I’d instal 100’s and resolve the battery equalization was the way to start the van. 250 amps is a lot of current on any wire less than 4-0 and I doubt you installed that. I was saying 500 amps is not impossible for the starter to draw not that you should fuse it for that!

 

[GaryBIS]

Hi,
Agree with RD that you could get quite high start currents and that putting in something like a 500 amp fuse means that the fuse won't be protecting the wire, so a short of the wire to ground could cause a fire.

The wire size calculator at BlueSea says a 2-0 wire (very beefy stuff) would be required for 250 amps even with a pretty short run:
http://circuitwizard.bluesea.com/

This is a good calculator that considers several factors in picking a wire size, including: the wire ampacity (max safe current), voltage drop, and the wire's insulation temperature rating. Its very easy to use and is meant for marine applications, which (I think) should be good for the RV world. It even tells you the rationale it used to pick the wire size.

This is what it provides for the 250 amp calculatioon above for 8 ft total length of wire:

Explain Results
The wire you select must be large enough to meet both ampacity and voltage drop requirements.

These are the wire sizes this circuit needs to meet each requirement:

Ampacity (Additional Derating Factors): No Additional Derating Factors were entered.
Ampacity (ABYC Standards Only): AWG 2/0
Voltage Drop (ABYC): AWG 2


I highly recommend this calculator, it meets the standards set by the ABYC (American Boat and Yacht Council). Some of the tables that you find on line cover voltage drop only and can result in unsafe wire sizes for short wire runs - cases where the wire ampacity is exceeded by a large margin even though the voltage drop would be OK. Others are intended for other applications and give wire sizes that are not very accurate for the RV/Boat world.

Gary

 

[catalin]

What's cheaper and better in the long run is a circuit breaker.

 

[medicineman4040]

.....and I'm actually trying (hard) to remember when I had a dead battery. 20 years ago maybe?
I'm just not so handy but will add one of the lithium jump packs for when AAA cannot be reached.

 

[SteveSS]

.....and I'm actually trying (hard) to remember when I had a dead battery. 20 years ago maybe?

You are so right. It's rare these days, now that dome and head lights turn themselves off. I just figure if I have equipment with the capability, I might as well make it functional.

 

[medicineman4040]

Yes and watch me have a dead battery soon enough now, probably cursed myself
I'll be visiting Harbor Freight soon enough to try and prevent that. I guess that's me
saying I've decided to remove the Battery Doc and put in the Projecta. Looks to be plug-n-play
since 3 wires on each, and each going to the same place, I just have to pick up some screw type
butt connectors big enough for 4 gauge cable

 

[Winston]

DC Breakers - - Polarity Conscious?

What's cheaper and better in the long run is a circuit breaker.

We utilize circuit breakers extensively in the DC side of our build. All of our breakers appear to be polarity sensitive - - which we don't perceive to be a problem in connection with the various comparatively low current DC branch circuits - - these breakers serve to protect against malfunctioning or shorted 'loads' only. A fault current will always and only be in one direction.

Our concern is with the breaker between our 280amp second alternator and the battery pack. The breaker is oriented to protect against too high a current FROM the alternator to the battery. But what if there is a short at the alternator or along the 2/0 cable between the alternator and the breaker?

This is the same problem most face when wiring their starter battery to the house battery. The fusing or breakers are intended to limit the current from the starter battery/alternator to the house battery. But what about those who intend on reversing the current, for example, when faced with a dead starter battery? Our research has not satisfied us that we're protected.

The solution? We're purchasing a larger fire extinguisher.

 

[MsNomer]

We've had a dead battery twice lately--battery will be replaced soon. In the meantime, it sure is handy to just connect the ground wire on the Surepower 1315 separator.

 

[PopeAng]

I had a dead battery on my car a few weeks ago. Left a rear door open that I thought was closed all the way. The dome light stayed on until the battery was drained. I called an Uber to come give me a jump start. Worked great! The most it would cost would be $8, but he "cancelled the trip" after he gave me the jump so I gave him the $ directly.

If I ever have a dead battery in town on my PM, I'll do that again.

 

[proeddie]

PA,

The rear door open is supposed to be on a timeout timer if you leave the door open... worth looking into. (now I have to check it out to be sure that's what it does)

My review of the HF jump starter gizmo I mentioned....don't waste the time to go down there! I got it, tested under load... pretty disappointing. Don;t know where all the good reviews came from... just my opinion, of course!