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How low can you go?


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My question is, with a dodgy alternator that is not producing any power, how low can the battery voltage drop before the bike becomes unrideable? As a project, I bought myself a 2001 FJS600 Silver Wing scooter with a known electrical fault which turns out to be a failed alternator. Thanks to COVID and living in New Zealand, I've had a frustrating time ordering bits and pieces to get the bike roadworthy and so far have failed to get an alternator shipped to me. As a result I've been riding the bike on battery power only. I put some LED head and taillights in to reduce the power drain but obviously the fuel pump, injectors and ignition system still need power to keep running, so what is the lowest voltage I can go to before they stop working? I've recently fitted a voltmeter (hey, I was also a VFR owner, I know about these things!) and have observed a battery resting voltage of 13.3V (freshly charged) which drops to 12.4V on starting, and then tracks slowly down from there. Yesterday it got down to 11.6V after 45 minutes riding and was running fine, but before the LEDs and VM, I managed to ride the bike to the point of dying (wouldn't idle/stalled plus warning lights for low oil pressure and excessive coolant temp), fortunately just as I arrived back home. 

 

For those who have had a voltmeter installed and have had the charging system fail. how low does the voltage drop before the bike stops?

 

 

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Interesting question. I do know that while cranking the bike over with a healthy battery (Fuel Pump fuse removed) battery volts dropped to 10.5v so I guess the EFI might be able to handle a couple volts or so less than that to allow for an ageing battery during start. I can't offer a definitive answer as to EFI cut-off voltage, and wether it's the Fuel Pump, Fuel Injectors, Ignition Coils or the ECM that throws in the towel first.

Cheers.:fing02:

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It is a combination of how much ampere your battery holds and how much watt you use...

 

Say the 12V battery is a brand spanking new one that holds 12Ah  (note that this drops as the battey gets older)

Say it holds 10Ah (ampere hour) from new

 

image.png.cf57d25ff36403bf7e8de810bf8c288a.png

 

 

You switch/took off all the lights and -this varies per motorcycle- say you use 150 watts (your cooling fan on 50% of the time.

 

150w / 12V = 12,5Ah

so expect 10Ah (battery)  /12,5 Ah (usage) = 80% of one hour, so ~45minutes

 

Use 300 watts?

300w / 12v = 25Ah

Battery 10Ah / 25Ah usuage = 40% of one hour so 24 minutes

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Don't know the answer,  but maybe an idea to get you through C19 would be a spare lithium battery that could be carried.  When the installed battery goes flat either jump it with the spare or swap it.  At least you wouldn't be stranded.

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On 7/25/2020 at 3:30 AM, Cogswell said:

Don't know the answer,  but maybe an idea to get you through C19 would be a spare lithium battery that could be carried.  When the installed battery goes flat either jump it with the spare or swap it.  At least you wouldn't be stranded.

Thanks for the suggestion. A swap is the only option here, using the lithium to jump start the bike with the main battery flattened will be temporary indeed! Jump starting a "healthy" bike with a flat battery is OK because once the engine is started the alternator is producing power. Not the case with a dead alternator (or RR for that matter). 

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On 7/24/2020 at 7:55 PM, Dutchy said:

It is a combination of how much ampere your battery holds and how much watt you use...

 

Say the 12V battery is a brand spanking new one that holds 12Ah  (note that this drops as the battey gets older)

Say it holds 10Ah (ampere hour) from new

 

image.png.cf57d25ff36403bf7e8de810bf8c288a.png

 

 

You switch/took off all the lights and -this varies per motorcycle- say you use 150 watts (your cooling fan on 50% of the time.

 

150w / 12V = 12,5Ah

so expect 10Ah (battery)  /12,5 Ah (usage) = 80% of one hour, so ~45minutes

 

Use 300 watts?

300w / 12v = 25Ah

Battery 10Ah / 25Ah usuage = 40% of one hour so 24 minutes

This looks remarkably like the calculations from the movie Apollo 13! Can I safely return home or will I run-out of oxygen on the side of the road...

  • Haha 3
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In the interests of science I rode the bike for nearly an hour today and parked up when the battery hit about 10.5V at which point all seemed well. I then left it to idle and kept an eye on the voltage. It finally started to misbehave at around 8.8V, but just prior to that the injectors and sparks were just fine. The LED hedlights started flickering around 9.5V. It was interesting to note that the rate of voltage drop increases dramatically the lower it goes. 

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I think there is another "backup plan" to use when your installed battery conks out. I carry a "jump-start" portable battery for the times I've left my headlights on accidentally. One summer while leaving town on a trip and sitting in very slow-moving traffic on I35 I saw a HD rider sitting on the side of freeway. So I stopped to see if I could help. She said her machine stalled and would not start. So I got my portable battery out, hooked it up, and she got her machine started. But it died as soon as the jump-starter was removed. So I suggested she could leave it connected and I'd follow her to a nearby exit so she could call for help. We were able to ride several miles (I cannot remember exactly how far it was) to an exit where, at the stop sigh at the end of the exit, here machine died and the portable battery was completely discharged.

As usual, YMMV.

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That's an intesting suggestions MaxSwell. The standard battery in the scooter is 11 Ah and gives just under an hour of riding time, so the duration of operation from a jump pack would depend on the comparative capacity of that. The few small units that I've looked at seem to be 8 to 12 Ah so would be pretty useful as a range extender! The only disadvantage will be when I use it, the now flat main battery is in the circuit and will be soaking up some of the charge from the jump pack. A sneaky switch (or a diode) would be in order to cut this out of the circuit. 

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