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Monday, October 11, 2010

10/11/2010 - Calling all DC geniuses

(caution: The following is a technical blog which may be of very little interest to you if you aren't fix-it minded. I'll return to my regular fluff pieces tomorrow.)

Okay, I've got a problem and I just KNOW there's someone out there with the answer at their fingertips. Ever since I was in the Navy I've been doing electrical work and I think I'm reasonably good at it. I even basically taught myself how Programmable Logic Controllers work way back in 1985. So, I think I have an edge over the average joe trying to take care of his electrical issues. At least that's what I used to think. But I'm stymied. Here's my problem:

I have two battery banks. One is the house bank and serves all the electrical needs on the boat except for starting the engine. The other bank is the starting battery and it's only job is to start the engine. The house bank is made up of 4 Trojan T-105 6VDC golf cart batteries connected in series-parallel so that I essentially have 2 large 12 VDC batteries. This bank has a total of around 250 usable amp-hours available for running the lights, autopilot, bilge pump, refrigerator, watermaker, stereo, 2-way radios, fans, inverter for charging the computer and cell phones, etc. The starting bank is one 12 VDC battery with a humongous amount of cold cranking amps but not a bunch of reserve power.

The batteries are separated in use by a selector switch which, when ON, sends starting juice only to the starter and house juice only to the distribution panel. When the switch is in COMBINE, the two banks are connected together. This is in case the starter battery gets run down and the house bank is needed to start the engine.

To keep these batteries charged they are connected to the alternator when the engine is running, the 160 watt solar array all the time, and the Xantrex charger when we're connected to shore power or the Honda generator. The Xantrex charger can charge up to 3 banks individually. I can program it for the type and size of the batteries it's dealing with and each bank can have its own charging regimen.

When charging with the solar cells or the alternator, the batteries are combined through a Blue Seas Automatic Charging Relay (ACR). The downside of this set-up is that both banks are charged the same way even though they have different requirements. Also, the ACR can interfere with the "smart" features of the Xantrex charger which is why Blue Seas issued a technical paper suggesting the ACR be disconnected when a smart charger is in use. I've been following this advice for some time.

So what's my problem?

Well, the problem is that I cannot keep the starting battery charged. I'm not really sure when the problem started but it was before I bought the new starting battery. I was having the same problem with the pair of group 27s I used to use for starting. I figured back then that it was just because the batteries were so old, so I replaced them with a new battery.

Here's what happens when I'm charging with the smart charger: The charger checks the batteries and applies charging voltage according to the state of discharge. If the battery is really low, it starts with a BULK charge where the battery is receiving high current. At some predetermined point, the charger changes to the ABSORPTION mode where the voltage remains constant but the current declines. Finally, when the battery is fully charged, the charger enters a FLOAT stage where the charger continues to deliver a lower level of voltage to maintain the battery in the fully charged state.

Well, this is all fine and good on the house bank which works just like it's supposed to. But the starting battery is another matter. Even though the Link 20 battery status meter shows a -36.5 amp-hour deficit, the voltage on the battery is up around 13.65 VDC so the charger thinks it's just fine and applies a FLOAT charge. But, as soon as I push the starter button on the engine control panel, it's clear that the battery is NOT fine as it can't even crank the engine over once. And, as if that wasn't enough, even with the charger connected, by morning the amp-hour deficit will be -37.1 Ah. Obviously there is something using battery juice even when the engine isn't being started, but that isn't even the issue. The charger should have absolutely no trouble keeping up with this miniscule draw but ti doesn't. It just lets the battery keep draining.

And it gets even weirder. Okay, maybe the Xantrex smart charger isn't so smart, but what about the simple alternator, or the solar panels? We can motor for 24 hours with the alternator merrily running along and when we get where we're going, you think the battery will crank the engine? NO! How about the house batteries? Oh they're just fat and happy and showing a 0 Ah deficit. Which is good, don't get me wrong, but I wish the starting battery would be so cooperative.

Last week, while we were at anchor, I fired up the generator and turned on the Xantrex so I could "equalize" the starting battery. Equalizing is when you force a fairly high voltage (around 16.5 VDC) through the battery to dissolve the sulfates off the lead plates. This is a way of revitalizing a battery that has been chronically undercharged or is just getting old. Every hour or so you take a specific gravity reading to check the state of charge. When all the cells are reading correctly, the battery is charged and you can stop equalizing. This looked promising at first. The voltage to the battery was high and the amp-hour deficit started to fall. After about 3 hours, all the cells were reading good. My hydrometer isn't good enough to get an accurate specific gravity reading, but they were all in the middle of the green zone. The Ah deficit was still -26.4 or something but I decided I'd run the generator long enough.

Next day I tried starting the engine and got the anemic Rrr-Rrrr... of a nearly dead battery for all my efforts.

So, I can't seem to charge my starting battery. The only time I've had any luck was when I disconnected the leads from the battery and connected a standard 6A automotive starter to it for a few hours. The next day the battery was able to start the engine. Reluctantly, but it started. I'm trying that again tonight and watching the Ah deficit to see if it returns to 0. It's not a regimen I want to follow but it may provide another clue.

Oh, and before you suggest it: the Xantrex requires the battery to have a minimum Ah rating of 60. My starting battery is rated at 90 Ah if I remember right.

So c'mon folks, help a poor sailor out. I'm hoping there's an old battery hand out there that'll write in and tell me some not-so-obvious mistake I made that could be easily remedied. Right now I'm flat out stumped. And I don't like it one little bit.

Okay, I just checked the Link 20. With the automotive charger connected to the starting battery, with the positive lead from the battery disconnected (except the one to the Link 20), after less than a half hour of charging, the battery status is now (supposedly) fully charged with an Ah surplus of 0.1 Ah. That doesn't make any sense at all to me. The game is afoot, Watson. Just to add fuel to the fire, I just now reconnected the main positive lead, the one that goes to the starter, back up to the battery and left the automotive charger on. We'll see what that does. Just for the record, the Link 20 shows me that the automotive charger is charging at 16.15 VDC on "High" (6 amps). I switched it to "Low" (2 amps) which stared out at 15.30 volts bit quickly dropped to 15.10 and was looking like it was going to just keep slowly dropping. Switched back to high and it's maintaining between 16.10 and 16.15 volts.

Help me if you can, I'm feeling down.


7 comments:

Wojtek said...

Steve, I'm not sure if I understand your connections. How about I tell you what works for me. As you know I have a very similar setup, same boat, the same house bank (4 x T-105s), which is located under the companionway ladder, same as you and a good cranking battery (740 CCA).
I have four charging sources:
1. Alternator
2. Xantrex charger
3. 150W solar panels array
4. Air-X wind generator
All four charging sources are connected directly to the Trojan house bank. The house bank and the start battery are connected together via a dual battery isolator switch - mine is called Redarc and happens to be manufactured here in Adelaide Australia but is available on eBay for around $100. The role of the isolator switch is to sense voltage in the charged bank (house bank in my case) and either parallel or separate the charging and the charged banks at a predetermined voltage levels. The spec says it will kick in at 13.2V and disconnect at 12.7V. My observations show 13.05V and 12.6V respectively.
So, say I have 12.4V in house bank and 12.8V in start. When I start the engine and the alternator starts pumping amps into the house bank the voltage increases and once reached 13.05V it parallels both banks and both are charged. When I stop the engine my electricity needs are absorbed from both banks until the voltage drops to 12.6V at which point the banks are separated by the switch. The power continues to be taken from the house bank and start battery seems to recover to 12.8-13.0V by itself. The same principle applies to other charging sources. In reality when I wake up on the boat the banks are already paralleled because solar panels are pumping amps.
I also have Xantrex Link1000 that manages the house bank and monitors voltage of the start battery.
My logic is such that when I sail during the day then solar panels and wind gen deliver enough power to keep the batteries topped up, at night it's only the wind gen. Night or day if there is no wind then I motor and the alternator does the job. While in marina with access to shore power the Xantrex charger kicks in.
So... what have you done differently?

Wojtek said...

Steve, you wouldn't believe it... I have just spent half an hour writing a response to your DC issues and when tried to post it it said it's too large and all is gone... I'll try to write again when I calm down...

Wojtek said...

Hey, look at that, it did get posted after all ;-)

Steve and Lulu said...

Thanks for the response Wojtek. My setup is almost identical to yours except that, where you have the Redarc, I have a Blue Seas Automatic Charging Relay which combines the batteries during charging but isolates them during engine cranking (unless I set my battery switch to COMBINE instead of ON). The ACR will not close if the one of the batteries is below 10.8 volts - presumably to keep whatever is draining the battery from draining the other battery as well.

Anonymous said...

Steve,

You've got 1 of 4 things going on: 1 Not charging, 2 discharging, 3 bad battery, or 4 bad connection between battery and starter. I'm sure you'll know at least most of what follows, but here goes. I believe going sequentially thru this makes the most sense.

Test charging by measuring voltage AT BATTERY TERMINALS while the battery is being charged. Should be 13-15V. If it is, the charging is most likely fine. If it's not, the voltage sensed for charging is probably not right (the charger thinks the voltage is higher than it actually is).

Once you're sure the charging system is working and the battery is fully charged, you can test for any external discharge by putting an ammeter in series between the battery cable and the positive terminal of the battery. Reading should be 0, but you may have a very small drain from your combiner (maybe 100 mV or so).

If it's not being externally discharged, I would get the battery load tested at your local NAPA. There's some tests for this you could also do aboard, but use the auto parts store if it's available.

If you're still not working, suspect your connections. I had the exact same symptom on Terrapin and fixed it by cleaning the physical mount of the starter to the engine which is part of the return path to the battery. Apparently, when on the house bank I had enough umph to overcome this, but on the starter battery I did not. My point there is to suspect everything in that path.

Good luck,

Rod
S/V Terrapin

Anonymous said...

Steve,
Sounds like you are checking all the battery charging systems correctly but have you considered a bad starter? On old 1954 ford panel truck I once owned, with a chrysler hemi in it, it went through three starter replacements because the starter would drain the battery and I could only get ut start if I jumped it. On the third starter fix the shop figured out that an internal bracket was cracked and would let the system short out when it got hot enough. The second battery would force enough amperage into the system to get things turning. Just thinking out loud.
Coyote

Steve and Lulu said...

Rod,
Thanks for the suggestions. My plan for tomorrow is to follow the path you laid out. Everything seems really tight but you never know.

Coyote,
I have contemplated the possibility of a bad starter and I just happen to have a spare on board. If Rod's suggestions don't do the trick, I may try swapping starters.

-Steve