my boat battery

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Post Options Post Options   Likes (0) Likes(0)   Quote Mr Moritz Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 2021 at 10:22am
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Originally posted by letsgetem letsgetem wrote:

Good stuff, thanks.

According to the motor specs (2018 90hp 4 stroke) - the alternator output is 50amps. THat appears a lot higher than the 2 stroke amps mentioned. 

One thing Im bothered by. THere doesnt appear to be a way of measuring the % charged of the battery. If there was, I would know whether the battery needs charging - and put the charger on it. Has anyone figured a way of measuring this? 


Just put a voltmeter across your battery terminals 12 Volts =flat 12.5 volts = Half charge   12.8 volts = Good fully charged battery.
Do this test when battery is at rest and at least 24 hours after charging cause charging pushes voltage up (14V )and takes about a day or so to settle down... 
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Post Options Post Options   Likes (0) Likes(0)   Quote letsgetem Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 2021 at 10:27am
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Oh damn I made another error - the 90hp motor charges at 35amps (not 50). 

The winch draws 83 amps. 
Anchor retrieval max rate is 41m/min. 
Anchor drop rate is 45m/min. (not free fall)

I havent measured the exact time the winch operates - next time Im out I will keep a record of this.

I have calculated roughly how much power should be used to drop and retrieve anchor - for my maximum anchoring, say 3 times at 25m depth (with twice the depth out). Its about 7.8AH.

Charging at 35Amps, 7.8AH would take 17.8mins to recharge. 

I would always run for longer than 17.8 mins (at high speed) coming back in - so, theoretically, the battery should have always been fully charged when returned to the ramp.

Whether the old battery failed because it was left not fully charged, or there was something wrong with it, I will never know. Interestingly, manufacturers warranty appears to be max 2 years ONLY! THats pathetic.

But, maybe things dont always work according to plan.
So, I will buy a Battery Comfort Indicator to ascertain when the battery needs charging; and a Ctek MXS 5.0 charger to charge it up fully. 
Can anyone comment on how this performs, or if any other battery charger would be better. 

THanks for all the points raised.


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Post Options Post Options   Likes (0) Likes(0)   Quote SaltyC Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 2021 at 11:10am
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The amps actually going into your battery are not controlled or set by what the charging output of your motor is. That is just the maximum that the motor can output.

The actual amps going into the battery is a function of the level of charge of the battery (the internal resistance of the battery increases as its state of charge goes up).

If your battery is above about 85% State of Charge (SoC) then the amps it will accept to charge are way lower than the theoretical max output of your charging device and usually this drops to about 10% of the battery capacity at 90% charge and keeps dropping as the SoC increases, so you may only be charging at 5.2 amps once the battery reaches 90 %. Ie it will take hours of running to get those last amps replaced and the battery back to full charge.

So, the net effect is that it is practically impossible to recharge your battery to 100% unless you spend hours trolling. As you do multiple trips you gradually decrease the level of chrge in a semi-cummulative manner. 

First trip starting at say, 100% you might end up at 92%. Next trip pulls the battery down further than the first because of the lower starting point and so is likely to end at lower recharge as well, say 90% etc etc etc as each trip gradually lowers the finishing SoC until the battery is more likely to fail, or you go trolling for 8 hours!

So, get a good battery charger, like the Ctek you mentioned and use it every time you return home. I don't personally like battery chargers left connected all the time as they can cause catastrophic damage if you get a dead short in the battery from a dead cell or thermal runaway in some AGM batteries. But a couple of days is all it needs each time.

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Interestingly, manufacturers warranty appears to be max 2 years ONLY! THats pathetic.

 That has been the case, even way back into the 60s.
 I meanted this above...


The Local auto electrician only has 2 rather 'oversized ' marine batteries in stock. Reason being he doesnt get ppl coming back because of early battery failure.

If batteries are treated right they last way beyond their warrantee periods.
Im thinking way back to my 1st car.. 63 triumph herald...and boat.. near 50yrs ago... I can not rem a battery ever failing under 6 to 10 yrs... Camaro went thru 3 batteries over near 40 yrs and 300,000 miles.. Bradford 1 over about 7 yrs and sold it with it still in. Big horn replaced the one we brought it with after about 4yrs, and the new one still fine 3/4yrs on. Even my jumper batteries last 10yrs plus.
 Tops always clean.. little over size to allow for extra loads, and always kept charged.
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Post Options Post Options   Likes (0) Likes(0)   Quote Sufishent Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 2021 at 11:34am
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Originally posted by Kandrew Kandrew wrote:

Originally posted by letsgetem letsgetem wrote:

Good stuff, thanks.

According to the motor specs (2018 90hp 4 stroke) - the alternator output is 50amps. THat appears a lot higher than the 2 stroke amps mentioned. 

One thing Im bothered by. THere doesnt appear to be a way of measuring the % charged of the battery. If there was, I would know whether the battery needs charging - and put the charger on it. Has anyone figured a way of measuring this? 
I use one of these, simple easy to install and read, I just hang mine of the side of the battery box where I can see it any time and my ctek smart charger plugs straight into it.

https://www.marine-deals.co.nz/smart-battery-chargers/ctek-comfort-indicator-eyelet-m8-8-4mm?gclid=EAIaIQobChMIpNvcrces8wIVgiQrCh2RBQDFEAQYASABEgJXePD_BwE

Although a great idea these are not foolproof. I have a multistage charger and always recharge the battery before a trip out. I also fitted one of these battery level indicators to my battery as extra awareness on battery charge. 

However I'm now having to change my battery as although the indicator is still showing green, the battery voltage has dropped to 11.6V and the battery won't hold a charge as I think a cell has gone. 
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Post Options Post Options   Likes (0) Likes(0)   Quote Bounty Hunter Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 2021 at 11:48am
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Originally posted by letsgetem letsgetem wrote:

Oh damn I made another error - the 90hp motor charges at 35amps (not 50). 

The winch draws 83 amps. 
Anchor retrieval max rate is 41m/min. 
Anchor drop rate is 45m/min. (not free fall)

I havent measured the exact time the winch operates - next time Im out I will keep a record of this.

I have calculated roughly how much power should be used to drop and retrieve anchor - for my maximum anchoring, say 3 times at 25m depth (with twice the depth out). Its about 7.8AH.

Charging at 35Amps, 7.8AH would take 17.8mins to recharge. 

I would always run for longer than 17.8 mins (at high speed) coming back in - so, theoretically, the battery should have always been fully charged when returned to the ramp.

Whether the old battery failed because it was left not fully charged, or there was something wrong with it, I will never know. Interestingly, manufacturers warranty appears to be max 2 years ONLY! THats pathetic.

But, maybe things dont always work according to plan.
So, I will buy a Battery Comfort Indicator to ascertain when the battery needs charging; and a Ctek MXS 5.0 charger to charge it up fully. 
Can anyone comment on how this performs, or if any other battery charger would be better. 

THanks for all the points raised.



your calculation on recharge time is flawed unfortunately

read SaltyC s post about declining acceptance rates as the battery comes up to full 
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Post Options Post Options   Likes (1) Likes(1)   Quote Kandrew Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 2021 at 2:34pm
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Originally posted by letsgetem letsgetem wrote:

Oh damn I made another error - the 90hp motor charges at 35amps (not 50). 

The winch draws 83 amps. 
Anchor retrieval max rate is 41m/min. 
Anchor drop rate is 45m/min. (not free fall)

I havent measured the exact time the winch operates - next time Im out I will keep a record of this.

I have calculated roughly how much power should be used to drop and retrieve anchor - for my maximum anchoring, say 3 times at 25m depth (with twice the depth out). Its about 7.8AH.

Charging at 35Amps, 7.8AH would take 17.8mins to recharge. 

I would always run for longer than 17.8 mins (at high speed) coming back in - so, theoretically, the battery should have always been fully charged when returned to the ramp.

Whether the old battery failed because it was left not fully charged, or there was something wrong with it, I will never know. Interestingly, manufacturers warranty appears to be max 2 years ONLY! THats pathetic.

But, maybe things dont always work according to plan.
So, I will buy a Battery Comfort Indicator to ascertain when the battery needs charging; and a Ctek MXS 5.0 charger to charge it up fully. 
Can anyone comment on how this performs, or if any other battery charger would be better. 

THanks for all the points raised.


Yep as I said mate 35 amp and about 400 plus watts of output, also has a water cooled regulator so I don’t know where this regulator overheating comes from. The Mercury 4S also run digital inductive ECM ignition not CDI. Some of the stuff posted on here can really confuse the solution.

Go to a larger battery if you want to but I would spend my money on a good smart charger. I have the same ctek charger pretty foolproof and I like the way the charger plugs straight into the comfort plug with out having to take the top of the battery box all the time and at a glance I can see where the battery at.

But there’s plenty of other chargers and voltmeters to look at depends on how much money you want to spend and how complicated system you want.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 2021 at 8:31am
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. I have a multistage charger and always recharge the battery before a trip out. I also fitted one of these battery level indicators to my battery as extra awareness on battery charge. 

However I'm now having to change my battery as although the indicator is still showing green, the battery voltage has dropped to 11.6V and the battery won't hold a charge as I think a cell has gone.


Chances are you may have , or have a voltage leak, maybe anything from salt/ dirt on the battery top between the terminals or in the isolation switch etc. A case of battery not being full charge when stored , and huge effect on battery life.

Quick test if battery going down/ cell dropping, and dont have a load tester...
Charge it up...leave 5 mins, note voltage. Then get an old incandescent car head lamp or something that pulls a lot current .. 12v pump, capstan/winch etc..run for 3 or 4 mins turn off wait couple minutes then check voltage... if good voltage will be same or very close to what was before loaded.
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Post Options Post Options   Likes (0) Likes(0)   Quote Mr Moritz Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 2021 at 9:45am
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Just a thought, I guess your battery is well secured in your boat. If there is any up or down movement the slamming could be damaging battery cells..
If you always do, what you've always done. You will always get, what you have always got
Henry Ford
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Hi, I've charger with a temperature sensor and sometimes i forget to connect it. Do the sensor make big diferance to the charging time?  
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Post Options Post Options   Likes (0) Likes(0)   Quote SaltyC Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 2021 at 10:30am
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The temp sensor is only there so the charger can stop charging if the battery temperature gets to high. Doesn't effect the charge rate, that is purely down to the batteries resistance.
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Originally posted by Steps Steps wrote:

. I have a multistage charger and always recharge the battery before a trip out. I also fitted one of these battery level indicators to my battery as extra awareness on battery charge. 

However I'm now having to change my battery as although the indicator is still showing green, the battery voltage has dropped to 11.6V and the battery won't hold a charge as I think a cell has gone.


Chances are you may have , or have a voltage leak, maybe anything from salt/ dirt on the battery top between the terminals or in the isolation switch etc. A case of battery not being full charge when stored , and huge effect on battery life.

Quick test if battery going down/ cell dropping, and dont have a load tester...
Charge it up...leave 5 mins, note voltage. Then get an old incandescent car head lamp or something that pulls a lot current .. 12v pump, capstan/winch etc..run for 3 or 4 mins turn off wait couple minutes then check voltage... if good voltage will be same or very close to what was before loaded.

Chances are you may have , or have a voltage leak, maybe anything from salt/ dirt on the battery top between the terminals or in the isolation switch - battery is disconnected after each trip so no stray currents and no salt/dirt on connections

battery not being full charge when stored - I have always charged before a trip and I will now charge after each trip

I used to check the battery voltage occasionally with a volt meter but used the CTEK led battery monitor system as my routine go to for monitoring battery level - after this issue it is now in the rubbish bin.


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Post Options Post Options   Likes (0) Likes(0)   Quote Big -Dave Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 2021 at 2:12pm
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Problem with charging a battery before a trip, is it can prop up a dying battery, long enough to get you out there and then go t*ts up..best to maintain if you are worried.

I admit i havent for years, use winches stereo etc, up to 3 months gaps between use as well, never had a flat battery unless the isolator was left on and the stereo or something.
I even switch to the house battery for a start up occasionally to see if it will still do the job.
you can't fix an idiot with duct tape, but it does muffle them for a while...
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Post Options Post Options   Likes (0) Likes(0)   Quote letsgetem Quote  Post ReplyReply Direct Link To This Post Posted: 06 Oct 2021 at 11:39am
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For the record -

I bought a charger Ctek MXS 5.0 (5 amps max) - $159 from Repco.
My boat battery started at 12v - thats pretty low, although it us still enough to start the motor. 12v has been called "flat", but its not really, ive read that 12v is about 25% charged.. 
THe Ctek charger, went thru 4 stages (of analysis stuff), then at stage 5, charged at higher amps until 80% charged - took 16 hours. Then went on stage 6, charging at lower amps until fully charged (12.8v) - took 10 hours. 

Ive permanently installed the Ctek input cable onto the battery - its plug (to connect the charger) hangs outside the battery box, that easy to use.

So - took 26 hours in total. THats quite a long time, presumably the 5amps is relatively low. Fine if you have plenty of time, but not if you want to get out soon.
I thought of getting a Ctek Comfort Connect Indicator, that shows voltage in 3 bands (red <12v, needs charging (although its not totally flat), orange 12.4-12.65v, partially charged, green >12.65v fully charged. But as I have battery voltage shown on my FF screen, thats easy and more precise, so I probably wont use the comfort Indicator.
NOw Im happy to get out there - now Covid regs allow boating - but Ive got a doctors appointment - BUGG.R.

Oh, and Im  charging my ute battery, only 12v after sitting doing nothing for 10 weeks.

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So - took 26 hours in total. THats quite a long time, presumably the 5amps is relatively low. Fine if you have plenty of time, but not if you want to get out soon.

 Modern agm batteries are NOT like our old school Pb car batteries... hence the new 'smart chargers'
They must be charged low and slow...
And as mentioned above.. must be tied down to reduce thumping.. they dont like that either.
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Post Options Post Options   Likes (0) Likes(0)   Quote letsgetem Quote  Post ReplyReply Direct Link To This Post Posted: 22 Nov 2021 at 11:43am
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Ive been studying how the battery charge changes between trips (ie motor charging).
At the end of the trip - 12.7v, thats about 100% charged.
Next day - 12.6v - interesting drop
then the voltage drops every day
after 5 days 12.5 about 90%
after 5 days 12.4 about 80%
etc - it appears to go down faster than I expect.

I have tried turning off the battery isolation switch, doesnt make any difference, so assume the drop is at the battery.

The battery is near new, 4 months old; and I cant see any dirt or salt buildup on the top of the battery.

So, I normally have the boat out of the water for around 14-21 days. The voltage should be about 12.4v when it goes for a spin and is recharged. I would prefer it to be fully charged.

So - two options - 

(a) I have a Ctek 5a charger, which if kept going all the time will keep the battery fully charged. 

(b) I could checking the voltage after each trip, and then leave it; and checking voltage around once a week, and fully charge before going fishing.

Im leaning towards option (a). Anyone see anything I should consider?

I imagine Covid lockdown has been bad for batteries, as they will have gone down too much and failed.


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I agree with Salty, Lead acid batteries in particular do not take kindly to being left in a low charge state or no charge / discharge for any length of time. Smart trickle chargers that slowly charge and discharge are the go. ( must get one for mine soon)If you have bad luck a battery can last two years, while many last 5 or more. In the old days every winter the automotive techs used to have phone calls from client with dead batteries, another issue is that batteries are at their worst when the weather is cold.
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Post Options Post Options   Likes (0) Likes(0)   Quote SaltyC Quote  Post ReplyReply Direct Link To This Post Posted: 22 Nov 2021 at 3:50pm
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Letsgetem, the discharge rate rate you are reporting over just a couple of weeks is not typical of a battery sitting unconnected, it is way too high.

I would suggest there is still something draining amps at low but steady rate even with the battery isolation switch off.

If you can, get your hands on a mutimeter with an amp clamp and check the actual amps being discharged with the isolation switch off.

If you can't find one locally somewhere, give me a yell and let me know where you are (Whangaparaoa?) and I will drop in with mine when up there next.


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Totally disconnect battery then check in 2 days as to what the discharge rate is,if nil,then isolator will be faulty,may need a clean as salt crystals making contact..
If battery is discharging when disconnect find receipt and take back.
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Post Options Post Options   Likes (0) Likes(0)   Quote MATTOO Quote  Post ReplyReply Direct Link To This Post Posted: 22 Nov 2021 at 4:46pm
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Salty has lead you to a very good area of watch.

There are others often not picked up.
One is when you have two batteries, house and start with a dvs link.
Start can drain the house because of evs priority.

However, one of the most unrecognised battery drain is heat. And I mean heat in the battery cabinet. Heat destroys batteries pretty fast.
You can check your environment by placing a thermoter to measure those temps. If it your boat sits in the driveway cooking in the sun, it may well be cooking your batteries to certain early death.

As salty said good regular maintenance s a key.
Your c-tek may do the job but not all chargers are equal in there ability to maintain batteries.
Some reading maybe helpful.
Just cruising in my now sweetas pimped out Southern 755 HT0!
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