How do I calculate amp usage?

[poorman9]

I working on my first bike build and trying to learn a bit more about XS electrical systems. I'm planning on buying a smaller than stock battery to run my bike (which will be kick only).

Here's some questions -

How can I calculator, or figure out how many amps my electrical system is using?
Could someone explain the theory behind this post from Pamcopete?

If I run a battery that is rated @ less amp hours than my bike needs, what will happen?

When my motorcycel is running, what exactly is the battery doing? Besides getting charged by the alternator. Is it running my lights, or is the engine doing that?

Thanks guys.

 

[jayel]

stock generator output is 11 amps at 2000 rpm
if to small a battery possibility of over charging
battery keep field coils in the gen charged to enable gen to supply electrical to other needs/charge battery

 

[xjwmx]

To figure out the amps you're using, select the things that you're running from pete's chart. Add the amps together.

The difference between that, and whatever Pete says the generator is putting out, is what available to charge your battery. Easiest just to try and see if it charges. If the battery got hot, it would mean it was overcharging. But if the voltage was actually regulated to 14.6 volts, I don't see how it could overcharge.

Amp-hours doesn't really apply. It's a measure of how long a battery will last if it's required to supply a certain amount of current during that time. The capacity of the battery in other words, on just one charge, not being continually charged.

As for what's running the things, it's the battery and generator together, really. But the useful way to think of it as the battery is charging from what's left over like I said earlier.

 

[poorman9]

Okay... Here's another question, If I get a lower amp battery (like 4Ah) do I need to worry about it being over charged?

I've read about guys on the board running .8amp batteries, how are those not being over charged by the generator?

 

[xjwmx]

Overcharging the way it's being used here relates to the internal resistance of the battery and how much current it draws, I think. In that sense, if the voltage is regulated to 14.6 volts, it can't overcharge. It can't be depending on the voltage to fall below that to limit the current it draws and save it from destruction Or if it can overcharge, then somebody explain how it's possible.

 

[poorman9]

I'm now more confused than I was before I posted my question. I was hoping to get the theory behind this, rather than just asking what size of battery can I run...but forget it, I'll just ask -

I'm running kick only, headlight, taillight, blinkers, and a couple little gauge lights, and also one of the Boyer blue box ignition systems. What's the smallest battery that will be safe and reliable to use?

The ridiculously small .8ah $10 battery?
Or maybe 5ah or 7ah?

 

[xjwmx]

.8 ah would power the rotor alone for about 20 min. .8 / (12/5 ) = .33 hr. Plus you have the cushion of the resistance of the brushes. I don't see why it wouldn't work. You should be able to go a lot smaller even. You only need to power the rotor a couple seconds with the battery. After that, the generator will keep the battery charged.

 

[retiredgentleman]

poorman9;

Here's a few points to keep in mind. The normal electrical load is about 8.7 amps ( rotor,ignition,tail light, headlight). When sitting at a stop light, about to turn, the brake light (foot on brake) and turn signals add another 3.8 amps. Total 12.5 amps. Note that there is no amps left for charging the battery.

The problem is that the alternator does not generate full output when idling at 1200 rpm. At 1200 rpm the alternator is generating around 8 to 9 amps. If you don't have your foot on the brake, and no turn signals flashing, the alternator can hold the voltage at about 13.2 to 13.5 volts. Now, apply the brake and turn signals...............the load goes up to 12.5 amps. The alternator is only putting out about 8.5 amps, so the other 4 amps has to come from the battery. The battery needs to have the capacity to supply the 4 amps.

Even with the battery supplying the 4 amps, the voltage will decrease and may get as low as 11.5 volts. Guess what................the stock flasher and bulbs will quit flashing at under 12 volts. Now you have a safety issue................you are going to turn at an intersection but your flashers just quit on you.

Here's my main point...................the alternator will not generate enough amps at 1200 rpm, so the battery capacity is your safety margin to add extra amps as needed.

Why try to skimp on the battery size...................use at least a 7 amp-hour battery to minimize problems and have a safe ride home.

Another point. Anything you can do to reduce electrical load is a real bonus on these bikes....................i.e. LED tail/brake light keeps your alternator and battery happy

 

[xjwmx]

A 1 ah battery could supply the missing 4A for 15 min. That means he could idle that long. OTOH, my analysis last night was just considering the rotor, which isn't realistic. Even the reg/rec itself probably presents quite a load. If people are using .8 and not having problems, go for it. I would always use a stock-size battery myself. So much more margin. I have stock everything in fact.

 

[retiredgentleman]

From post #6:
I'm running kick only, headlight, taillight, blinkers, and a couple little gauge lights, and also one of the Boyer blue box ignition systems. What's the smallest battery that will be safe and reliable to use?

poorman9 was asking for the smallest battery that is safe and reliable. Safety is a subjective term. Some riders that use 0.8 amp hour batteries may also run with no front brake and no turn signals. If a rider feels safe running with the bare minimum equipment, then that is their choice.

I prefer to have a safety margin.

 

[xjwmx]

well, the only way to find out really is by playing with it. We don't know what the successful .8 ah'ers are running. I have no need for a smaller than normal battery, myself. I think there are lots of threads about smallest batteries.

What does a typical PM system deliver? They're using a cap which is working like a super, super weak battery. So in that comparison the battery in our system just powers the rotor

 

[poorman9]

Awesome guys! retiredgentleman, that's exactly the kind of information I was looking for.

I think I'll go with a 5ah battery, which will work well for the stripped down style bike I'm building and should give me plenty of power while I sit at a stop light with my blinkers, headlights, and taillights on for 30 minutes...

Anybody use any of these UPG brand batteries?

Here's a 5ah, 12v, AGM battery that's 3.5"x2.75"x4" and only 3lbs that sells for $8.58

http://www.1000bulbs.com/product/58477/BAT-UB1250F2.html

 

[XSLeo]

The PMA's put out about the same to a couple amps more. The main advantage is with out the battery power rotor the 5 amps or so the rotor uses is availible for charging.
The lack of the battery powered rotor is why they can run without a bettery, just a capacitor.
On the stock system everything runs off the battery, the alternator just replaces the electricity the battery loses to run the bike. In the stock system it's a bare minimum charging to just keep the bike running. If it gets weak, bad charging, or the load gets bigger , dirty conections, then the battery gets discharged.
Leo

 

[xjwmx]

the 5 amps or so the rotor uses

I think the rotor is using only about 1.25 amps. ~3 ohms for each brush and 5 for the rotor and 14.6 for the voltage. The dirtier the connections, the less it uses

 

[kzelisko]

I think the rotor is using only about 1.25 amps. ~3 ohms for each brush and 5 for the rotor and 14.6 for the voltage. The dirtier the connections, the less it uses

Bare in mind that while resistance with a standard meter might be 11 ohms, it could be very different while the alternator is functioning. The only way to be sure about that would be to hook up and ammeter and see what the alternator pulls while running

 

[xjwmx]

^The only thing that could change the resistance would be if it got hot, in which case it would go up, meaning even less current....

 

[retiredgentleman]

Actually the brushes have very little resistance.................maybe 0.5 ohms each. I have measured the current drawn by the rotor; its about 2 amps with the engine at 1200 rpm.

When you rev the engine to 3000 rpm, the regulator reduces the rotor current flow down to around 1 amp.

Everybody likes to badmouth the brushes, but in reality they work quite well. They do wear, with the outer wearing faster. Each year I just check the length and clean up the slip rings. This is my fifth year with the bike, and I have never had any charging problems.

Whenever you hear riders saying they have to put a battery charger (tender) on their bikes during riding season, it just tells me they are not maintaining the charging system.