How do I test charging with only a Multimeter?

[donradcliffe]

Mark is porbably going to have more electrical adventures getting across the Pacific, but the multimeter tests he ran can tell a lot about his system:

Check voltage before, during and after charging.
The 'Lights' mentioned are 4 cabin lights.

Results:

12.32 volts at 6.59pm
Started engine 7pm.
13.30 volts 7.01pm
13.55 volts 7.30pm with lights on 13.38 volts
13.61 volts 7.50pm with lights on 13.44 volts
13.60 volts at 8pm with lights on 13.42 volts
Stopped engine 8.01pm
12.77 volts with lights on 12.57 volts
12.68 volts 8.06 after a drain to remove surface charge.

So what is his multimeter saying:

1. His alternator works, bringing the batteries up to their regulated voltage of 13.6 volts after half an hour's charging.

2. The regulator is likely a single stage internal type, which maintains the output terminal of the regulator at 13.6 to 13.8 volts (you can put the multimeter directly on the output terminal to get a more accurate value).

3. The voltage reading at the battery comes up to 13.55 volts by 730 pm, which is less than the voltage at the alternator due to the voltage drop through the charging wire. This is the end of the bulk phase of charging, where the batteries are taking the entire potential output of the alternator.

4. The battery voltage slowly rises from 13.55v at 730 pm to 13.60v at 800 pm during the acceptance phase of the charging cycle. The voltage at the alternator is held constant by the regulator, and the current going into the batteries slowly drops over time. The small increase in voltage at the battery terminals is because the voltage drop in the wire (and any connections) from the alternator to the battery is decreasing as the charging current decreases.

5. The 0.17 volt decrease in battery voltage when the lights are turned on is most likely due to the voltage loss in the wire from the alternator to the battery. The regulator is boosting the alternator output current to compensate for the light load, and the increased current leads to a larger voltage sag from the alternator to the battery. Here the multimeter test is telling us something else--this voltage sag seems too big for the light load(which I assume to be up to 5 amps), and its time for some more work.

First, clean all the connections between the alternator and the battery (both hot side and ground). It may not change things, but it certainly can't hurt and doesnt cost anything. You can also measure the voltage drop across each connection while the battery is being charged at high current--the dirtier connections will have the highest voltage drop.

Second, measure the alternator output terminal voltage to see if it changes when the lights go on. It should in the first half hour of charging, when the alternator is putting out all it can, and its output is shared by the light load and the battery charging load,. However, if you have been charging for more than 45 minutes (well into the acceptance phase where the alternator is controlled by the regulator) and the alternator voltage drops by more than 0.05v when the lights come on, then you have a dodgy regulator.

Finally, determine if the wire from the alternator to the battery is big enough (#4 or lower) for 80 amps. If it is too small, which is a common problem in production and charter boats, put this on your list for when you get across the pond.

As pointed out in other posts, your battery bank is to small for sustained performance. Every time you discharge and recharge the batteries they are losing capacity. This loss is non-linear, so that 4 discharge cycles which take out 25% of the bank's capacity will cause less capacity loss than 2 discharge cycles which take out 50%. Thus, to nurse your batteries across the Pacific, you are better off charging for 1/2 hour every 6 hours, than charging for an hour every 12 hours, or especially 2 hours every 24 hours. Discharge batteries also suphate with time, and since you don't have the capacity for equalizing the batteries, the more frequent charging regime will also help with that problem.

One final comment--while you are in the initial bulk phase (alternator maxed out), the battery voltage and the charging current will vary with engine rpm. The battery voltage will gradually be going up anyway, but you will probably be able to see a higher voltage as you bring the engine rpm up. With practice you will find a 'sweet spot' probably between 1000 and 1500 rpm where the engine is smooth, the alternator output is a high percentage of maximum, and you aren't burning too much fuel. The engine gurus are going to jump in and tell you that running at a fast idle is not good for the engine, but you won't wear it out in a few hundred hours, and you can put in a fancy regulator, bigger batteries, solar panels, etc. when you get back to the first world.

[MarkJ]

Originally Posted by donradcliffe

but you won't wear it out in a few hundred hours, and you can put in a fancy regulator, bigger batteries, solar panels, etc. when you get back to the first world.

Dear Don,

Thanks so much for such an insightful post. Its taken a lot of work and analysis from you and is much appreciated

Originally Posted by CaptainJeff

. For the price of only 800 dresses, ...
Fair Winds,
Jeff

Some girl is gunna be wondering about naked for a loooong time

Mark

[Therapy]

If you are gonna run the engine then why not just put it in gear?
Fuel use?

[Ex-Calif]

Glad it worked out. As I said before you can buy the three idiot lights attached to a cigarette lighter plug for about $10.

[DeepFrz]

Understanding the Three-Stage Regulator

Here is an article that talks about 3 stage regulators and automotive types. Here is one little snippet out of it. It might be a worthwhile read before you step off the "Left Coast".

How does the performance of a standard regulator compare to that of a three-stage regulator? Not very well if the regulator is an automotive unit with a cut-out voltage around 13.8 volts. A 13.8-volt regulator may be satisfactory on a powerboat, but it does a lousy job of charging sailboat batteries. Raising the battery charge level from 50 percent to 90 percent with a 13.8-volt charging source takes more than five hours.

You can find it here:
Panbo: The Marine Electronics Weblog: Electrical systems

way down near the bottom of the page.

[Vasco]

You hit it right on the head, Deep, whether the idiot lights work or not an automotive type set-up will not work in the long run.

[MarkJ]

Originally Posted by DeepFrz

way down near the bottom of the page.

I scrolled down a $100,000 refit!

Food is the top prority at the moment (if you get the drift )

Thanks for your help and ideas, everyone has been great

[bridgecastle]

Hi Mark,
You may use a multimeter to confirm that your batteries have a charge/being charged. You may also use a multimeter to get an open circuit voltage on your batteries. To do this, rest your batteries for about 12 hours. That is, do not have a load on your batteries nor put a charge to them for about 12 hours but prefer 24. To get an open circuit voltage for wet cells (Gel cell & AGM) measure across + and - post of the battery.
A rested battery should give you the following,
100% charge 12.6v (12.8),
75% = 12.4v (12.6),
50% = 12.2v (12.4),
25% = 12.0v (12.2),
00% = 11.8v (12.0).
Your batteries are taking a full charge very quickly, they are probably nfg as they are only taking a surface charge.
Get yourself a hydrometer, this will tell you which individual cells are suspect.
have the batteries load tested, this will confirm good/nfg.
Good luck.

[Alan Wheeler]

Your batteries are taking a full charge very quickly, they are probably nfg as they are only taking a surface charge.

Not quite. You have to balance the fact that there is only 110Ahr of capacity with the size of Alternator. Sure the bank will never be fully charged but the voltage Mark is seeing in those times would be quite normal for a bank and Alt of those sizes.

[hellosailor]

How commonly are folks seeing 13.8V from internally regulated alternators in the first place?!

A 35 year old Delcotron puts out 14.3-14.4 volts, in that narrow a range, and is considered defective if it goes outside that range by even 1/10th of a volt. 14.4 volts is the target range for pretty much all modern (20-30 years) alternators once they are ABOVE IDLE SPEED. Even my antique riceburner with an obsolete Japanese alternator puts out 14.3 at "highway" rpms.

If a lot of folks are seeing 13.8 and considering that to be normal during initial (bulk) charging? I'd suggest there is something wrong, or at least, not running up to speed.

Went through that a couple of years ago. We had a custom pulley machined for an alternator, according to mfr specs, and damn near doubled the output of it because the POS that was bolted onto it, just wasn't meant for that application.

[DeepFrz]

A quick search of the web shows that auto manufacturers set the voltage in accordance with the type of battery chemistry used in their cars, or dogma...<gr>

It's difficult to say what auto manufacturers use as a set point for their voltage regulators. For marine batteries I guess it's best to follow the battery manufacturers recommendations and use a 3 stage (at least) regulator.

Here is an interesting read.

CAR BATTERIES ARE NOT 12 VOLTS

[Therapy]

Originally Posted by DeepFrz

Here is an interesting read.

CAR BATTERIES ARE NOT 12 VOLTS

Very interesting, though a little dated.

[DeepFrz]

Actually it's a lot dated, but I think it highlights the need to follow battery manufacturer's recommendations. Although I seem to remember reading on the forum (this or elsewhere, not sure) that Lifeline tech. support recommended a different float voltage than what is posted on the website. Not sure of the circumstances but it was about 1/10 volt higher than the website recommendations. Maybe someone else remembers that post and could give us a referral to the thread.

[Josh H]

Nobody mentioned it but i thought that i will toss it out here... When using the Amp meter part of your multimeter make sure that you are placing the meter "in series" with the battery and the loads, not "in parallel" like you do when you measure voltage. (see drawing below)



The battery is on the left, the load is the box on the right, the meter is the circle in the middle, and everything is connected to ground at the bottom.