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Sentry Battery Chargers
October 30, 2010
10:14 am
Steve Pooler
Forum Posts: 1127
Member Since:
November 12, 2008
sp_UserOfflineSmall Offline

Greetings All,** you don't have permission to see this link **

It seems that there is some confusion/misconceptions about Sentry battery chargers...Actually all chargers, but with the recent years of newer technology in battery charging (switch mode or multi stage) when compared to the older (time proven) technology of Sentry & others being Ferro Resonant transformer type...below is a bunch of excerpts from a forum I visit...

All of this below is repeated info folks...Just wanted to try to centralize it...It's copies of posts & PM's I made & received...Hopefully it will answer some questions & dispel some BS... The names have been left out to protect the...
Any questions...I'll try to answer...


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I have a Sentry G330-3NCL charger that is connected to each of the two 32 volt battery banks on my boat. Each bank is comprised of 4 8 volt batteries, the size of a group "819" per the Deka catalog. They are Interstates bought a little over 3 years ago by the prior owner, so I do not know exactly what they are. The starboard bank just starts the starboard engine (it once also started the generator, but not anymore). The port bank starts the port engine and serves as the house bank.

The engines have always started instantly. But a few months ago, the starboard bank stopped functioning, and a load test determined that it was spent... it would show a surface charge of 34 volts but immediately go into the low 20s with any load. All four batteries upon individual testing were equally bad. By using the parallel start, the engine as usual starts instantly.

The batteries have always required routine watering at least once a month, and the PO says that was the case for the prior set as well.

Due to a misunderstanding amongst crew, I went a couple of months once without watering and found the tops of the plates exposed on both banks. The starboard bank went out a few months later. The boat was in almost daily use during this time.

The charger has always come on soon after any load is put on the system, well before any draw-down would dictate charging, even when the batteries were virtually new. This has always bothered me. Does this thing charge both banks or just the banks it senses are in need? I have this sense that the starboard bank is getting over charged. The alternator checks out fine, but perhaps needs another look, regulation wise. One thing that is curious is that both banks need the same amount of watering, though.

Anyway, any advice you have would be greatly appreciated. I am considering changing the starboard bank to the physically smaller, but seemingly more cranking-oriented 8v195s. Would this be an issue with this charger?
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Your Sentry charger is only smart enough to know when a battery or bank is below 34.5 volts (cut in) or above 37.5 (cut out)...At least that is what it should be set for, and there are factors that can affect this (I'll explain in a minute)

Anything above 37.5 and the 32 volt bank of batteries will sulfate & boil the electrolyte out of the cells causing the the need to fill as you mention.

The original circuit board (which likely has been replaced by now) only had 1 (red) sensor lead that was typically connected to the house bank (inside the charger) because it was the bank that got discharged the most....The thought then was the bank with the least charge would trigger the charger (at 34.4 volts) and would also accept the most amperage...others with no sensor would hover where they were until the lowest bank would come up to their level (voltage) and then all would come up to the cutoff of 37.5 together (yes it always charges all banks connected when running)

The replacement boards now sense up to 4 banks (your charger only has 3 outputs) and came with all 4 red sensor leads piggy backed together to sense one bank just as the older model did...It was then up to the installer to separate those leads as needed...In your case with two banks...You would piggy back 2 leads on one bank & two on the other (each of the 4 leads does need to be connected to a battery or else the one not connected will see it as NO voltage and never allow the charger shut off)
In this scenario...Either bank can bring the charger on if it drops below 34.5 volts, and it will still charge all banks until all reach 37.5 volts, then it shuts the charger off...

Now that I have explained how it should work...Lets talk about what could be happening...

You may well have an old board, or one that is out of calibration...You can re-calibrate, but if that board is many years old...The adjustment potentiometers may be dirty or corroded, and not hold calibration well...
Sometimes it's just better to start out with a new board because it take some time & effort to re-calibrate and find out it hold won't calibration...

The other point I'd like to make is the sensor leads are connected inside the charger (and a new board will come calibrated properly)...But if there is a voltage drop (in the charging leads) between the charger & the bank of batteries...The board will see this at the charger end as a lower voltage, and continue charging past the 37.5 sulfate point the batteries may have reached at the other end...It will also cause the charger to see lower voltage when off, and thus cause it to come on prematurely.

I always like to calibrate the board with my meter (digital of course) connected to the batteries, not inside the charger, to compensate for any voltage drop in the wire....This way I can see where the batteries are actually at the battery & not in the charger.

If you want to calibrate...I can walk you thru it, but it can be tedious to get right...A helper is useful to turn on & off loads while you watch the meter & make the adjustments...

If I were you...I'd first open the charger & see how many RED sensor leads are connected to outputs...For testing they all need to be on the bank you are testing so that you don't need to measure & watch more than one bank at a time.
Then go to the bank in question & watch it with your meter...See what voltage the charger shuts off at...If it's above 37.5...That will explain the water loss.

The Sentry is a very good charger that is built like a tank...All parts are still available, and it is the only charger I know of that shuts completely off...

Oh and...The Sentry doesn't care what size the batteries are...Only their voltage (which it can be calibrated to meet any type's requirements)...that's all that batteries care about too...
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Yes, The cut in & cut out can be adjusted with potentiometers on the Sentry board...It's a bit tedious to do so, and a helper is good for turning on & off loads while you are watching the meter & making the adjustments.

On some Hatteras's I used to do it myself by utilizing the main port (house) battery switch which was located near the charger...I'd first turn on something of heavy load, and then use the switch to raise or lower the voltage with that load...If you want to do it quicker is where a helper is good...With the switch on you can get them to say flush a head (high draw) to lower bank voltage.

The trick is to avoid the sulfation point & thus the boiling of electrolyte out, So the cut off is most important...Then as batteries age and maybe won't maintain a higher standing voltage, the cut in can be lowered to keep the charger from short cycling and to extend useable battery life...I'm just quoting Sentry's cut in & cut out voltage recommendations to keep good lead acid batteries fully charged....Which are as follows:

12 volt system...12.8/13.9
24 volt system...25.5/28.0
32 volt system...34.5/37.5

If a battery or bank of batteries won't stay within the above parameters with no load...Then it's getting to be time to consider replacement/s...

Edit: Also, if you are only charging 12 volt batteries to 13.6...They are not getting fully charged, and will not maintain a 12.8 standing voltage with no load...Charged to 13.9 a good 12 volt battery will settle around 13.0/13.2 volts, or about 2.1/2.2 volts per cell.
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I know we have been around about this but....I still say you have a problem on your boat...Maybe with the charger...Maybe with the wiring or batteries/type...

But when operating/setup correctly, a Sentry WILL NOT BOIL BATTERIES....That's the whole purpose of it's shut completely off design, and Sentry's have been working well in the field for many...For many, many, years...

Just get your Sentry problem fixed...You can do that with a Sentry...And it's best to do it where it lives and is connected.
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All Sentry control circuit boards have cut in & differential pots...The trick is to know what you are looking for & where to look...

Just because the PO had the same problem does not make it right that you should need to water your batteries every three weeks...

First I suggest you verify your exact cut in & cut out voltages with a digital volt meter on each of the charger's outputs..Better yet at each battery bank...Even better than that at both places...Best is to measure it on the way up, down & at rest with the charger off (both places)
This way you will know how much voltage drop is in the wire & connections to the battery banks...

If any bank goes above 37.5 volts (at the batteries) for any long period of time...You have a problem & the electrolyte will boil out...The longer that period over 37.5...The quicker the electrolyte boils out.

Voltage drop thru the wire or connections can cause the control circuit to see incorrect voltage and thus not cut in or out at the proper battery voltage....The charger can be calibrated to compensate for this voltage drop, but often a service guy or owner will just install a new control circuit board (factory calibrated) and call it done...This most often solves nothing...

This is why I say the setup/checkout/service should be done on the boat and not on a bench where the charger does not live with the wiring used.

Any of the newer chargers that sense voltage and cut back at certain voltages may have the same problem/s if installed to the same wiring that a Sentry was removed from...Because the same boat/voltage drop problem still exists....If the charger (any charger) cannot get an accurate reading it's going to affect it's operational modes and how it relates/reacts to the batteries it's charging....

The good thing about a Sentry is it can be calibrated to that problem where many of the newer switch mode chargers can not.

Of course it's best to address the voltage drop problem, but many times that might cost more than calibration of the Sentry...
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Most all FR chargers (by nature of the design) including the Sentry, will taper back amperage as battery voltage rises...Problem with that is...Most will not shut completely off like the Sentry, and will continue with a trickle charge...However the voltage will also continue to rise above 37.5 with that trickle amperage and boil the electrolyte...

A Sentry will do the same if left in the Manual mode (front switch on manual) which bypasses the control circuit that shuts it off automatically...Or if the control circuit board fails, but you will notice that the Sentry never shuts off.

If you still have the Sentry...Why not clean up the wiring in question & put it back ?....All parts are still available for it including the control circuit board.

I will say the same to you...Check the voltage at the batteries with a digital meter...Chances are you are close to, or over that 37.5 sulfate point for long periods causing rapid electrolyte evaporation and overheated batteries.
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Steve I have been through the drill a few times now.
The charger puts out about 39 volts when charging and seems to cut in about 36.4 volts. Measured at the batteries. Both way too high, obviously. There is about a half volt drop over to the port bank vs the starboard where the charger is. There are three sensing leads to the port side (house bank) and one to the starboard. I will measure the voltage at the charger tomorrow, but the 39 volts is obviously a problem. I cannot find pots anywhere inside, I will try one more time; I do know my way around circuit boards a bit.

All the wiring is quite robust and connections in good shape.
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So yes you do have a problem & no wonder you need to add water every three weeks with a cutout of 39+ volts...
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We can talk about what to do about it all but I will say the first thing is to NOT start adjusting...That's why I don't offer where the pots are...First thing is to find out what else is happening with the voltage & the sensor wires...


Is it cooking one bank or more? I had a ferro-resonant charger that worked OK on one bank, but the 2nd and 3rd banks charging terminals just ran whenever the primary bank was getting charged. Only one bank had the sensor wires to determine when to charge and when to stop. Since the 2nd bank was just a starting bank, it was always close to full charge from the alternator when running and the charger just cooked it. It has since been disconnected since my Outback inverter is also a very smart charger that works well.
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What you are saying is possible (more on that in a minute) but first I'd like to say that unless a charger has more than one charging source inside it's box (individual transformers or charging boards)...That could be true of any charger...It would also make them either very heavy (transformers) very expensive, or both...Unless the output amperage to each bank was very small (2.5 to 5 amps each) such as in small bass/flats boat chargers that are essentially 2 or 3 completely different chargers mounted in one box and are not built to keep up with loads, but to replace charge after the boat returns to shore.

Though I'm not familiar with your Outback inverter/charger...Most inverter chargers do utilize 3 or 4 stage smart charging technology...However most also only have one output meant to recharge the battery bank they draw from to make the A/C voltage...The smart technology is very good at replenishing a large deeply discharged bank of batteries quickly with their high output amperage, and the fact that they can vary output amperage at differing voltages throughout the charge cycle...
But to use most inverters as a multi bank charger, the output charge must be split up somehow, which is usually done outside the charger with either a battery isolator (that has major voltage drop across it's blocking diodes) or a charging relay that works on voltage to tell it when to close to allow the other bank/s to receive charge...These charging relays (battery combiners) work well, but one is needed for transfer of charge to each bank as voltage rises...They are also another thing to calibrate correctly with the use of a smart chargers varying output voltage.

Ferro Resonant chargers such as the Sentry are still a great option for regular everyday use...Though by nature of design they will not replenish to full charge a large bank as quickly as a smart charger of the same amperage output rating when it is used as a inverter supply and is discharged deeply.

Ferro Resonant chargers such as Sentry have no way to control output voltage (at least not as a smart charger technology does)...It (voltage) rises with the percentage of charge in the battery as it's replenished...It also tapers back amperage output as that battery rises in charge...Or another way to look at is...Batteries will only accept a given amount of amperage related to it's individual charge state...A (good) dead battery can & will accept more charge amperage than a half charged battery, and a close to fully charged battery will accept less than that....Therefore as the (good) battery/s charge...The charger tapers back output.
In a multi bank situation...The lowest bank will accept the most charge until it reaches the level of the next bank, then those two banks will share that same charge until they reach the level of the next bank etc...Until all banks reach fully charged and the amperage output of the ferroresonant charger is then very low overall...At this point if it is a Sentry that can sense voltage...It will shut off....If not a Sentry it will continue to trickle charge at low amperage but the voltage will climb above the battery sulfate point of boiling the electrolyte out of the cells...
During this process...This also happens inside each battery, as a battery is no more than a combo of batteries (known as cells) housed in one box and tied together internally.

As far as the Sentry only sensing one bank...Yes that was true 20 or so years ago when the board only had one sensing lead to connect to one bank.
Most of those old boards have likely been replaced by now (think I still have a few on the truck) but the newer Sentrys (G series & up) and their replacement boards all come with 4 sensor leads...But it's up to the installer to separate those leads and attach them to the banks connected to the charger (Sentry has no way to know how many banks or which output posts the installer is connecting the banks to)

To be truthful though...Even if all sensors are connected...The board only knows if any bank (one or more) is below the cut in point (charger will come on) or if all banks have reached the cut off point (charger won't shut off until all banks reach that point)

This design requires that all banks/batteries be in good condition, as any one bad can throw a monkey wrench into the works...Causing the charger to either short cycle, or stay on longer than it should trying to finish off that last battery that won't quite come up to cut off voltage...Sentry can also address this type of problem with low, med & high output taps from the transformer that can raise the finish amperage to help that one bank over the hump to full voltage and cut off.

All of this description is providing there is no load on the batteries...Constant load on any battery bank can hold the charger on, matching that load (because the load causes the banks voltage to drop below cut in) but usually that would need to be a fairly large load to keep the charger from overcoming it and shutting off...Also a weak battery in the bank that is supplying the load can affect this.


Thanks Steve,

I am the one with the 39+volts cranking out, and around 35 volts for cut in. There is no measurable voltage drop from charger to batteries, that is. the voltages read virtually the same at the charger as they do at the batteries. I have the four sensor lead set up. Three are connected to one bank and one to the other. There are what appear to be two little pots on the circuit board, but the way it is mounted and designed would make it very difficult to get in there and adjust them. My port bank is pretty much gone now (works OK for house purposes but won't turn the engine). So, I really need to solve this problem while I am in the process of putting in new batteries. Since it is the house bank, I lean more towards replacing them with the Rolls or at least the 819s in there, rather than 8v195s.


When we bought our 60C the battery water was vanishing to the point of needing topping up every couple months or so.
Fresh sensor wiring and a fresh ground wire between (all five of) the more water.
I have a smart charger for the 32V banks on my wish list, but honestly, the sentry charger is doing it's job well enough.


Until you get those batteries replaced, It doesn't make much sense to calibrate now because you will need to do it all again when you do get the new batteries installed...

Yes the pots are a little awkward to get to but with the right small (flat blade) screwdriver it's not too bad...What model is your charger, is it a C or G series ?

With the G series you can (with a 1/4" nut driver) remove the 4 screws holding the front switch/meter plate...You can then swing it out of the way & tie it back while still connected...This should give you a bit better access...If it's the older C series you can do the same with the 2 slotted screws at the top of the meter plate...

It's important to put all the sensor leads on the battery/bank/output you are calibrating for...This way it's only looking at one source, and you don't get confused if one bank is a few tenths different that the other...

The pots are not cut in & cut out per say...One is differential, and depending on the board C or G series the other is either cut in, or cut out...
You have a G series board so when you get the new batteries in...let me know...
I can never remember which is which & always have to pull the book out of the truck to refresh my memory...I don't have a copy here (I should) but thats where I always need it most, and I have not been able to find a PDF copy on Cruisair's site...Even in the dealer access only area....

Like I said...I'll pull it out when you get ready...

For now if you want...I would put all the leads together & connect them to the good stb bank...Then check cut in & out voltage on that starboard bank...It could be that the bad batts in the port side are throwing everything off...But it will tell you what the board is calibrated for, and If it charges those to above 37.5 then you may want to turn the charger off for part of the day to keep from overcharging them until you replace the port.

I am correct in assuming that the charger does shut off right ?
If not we need to test the board by jumping battery power to one of the blue ignition cut out wires to insure the triac on the board is not bad...With 32 volts to either of the blue wires (one for each engine) the charger should cut off...If not we need to get you a new board.