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Amp hours and voltage

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Jeremy View Drop Down
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    Posted: 01 July 2018 at 08:12
Hi Guys,
I have the standard 415 battery setup (2 x 165ah batteries so 330ah) and Ive recently installed a BEP battery monitor. 
Over winter Ive found we are drawing a lot more power over night with fewer day light hours and less effectiveness in solar charging. By morning we are getting very low on power, the displays are showing approx 11.9V in the house system. But here's the issue. The BEP monitor is showing we are down to 11.9V but also saying that we have 300ah remaining. Thats 90% charged! Ive heard these AGM batteries are good till about 50% charge. So im rushing to get the motor started because im seeing 11.9V.
Im not sure of the relationship between V and Ah, who should I trust? My batteries are the original ones (2014) Maybe they're nearing their use by date?

Thanks heaps!
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charentebob View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote charentebob Quote  Post ReplyReply Direct Link To This Post Posted: 01 July 2018 at 10:07
Does the BEP also monitor the starter battery? You mention rushing to start the engine as the battery was 11.6v. Some monitors assume 100% when fully charged at installation even if though the batteries are not new. It may be worth having an electrician check once you have charged well to have a datum point. My batteries are 5 years old & OK. Last year I installed a Victron digital amp meter & Colour display. This displays the solar, engine charging & shore power inputs & discharge. Now I can ensure that the batteries never get below 80% which will hopefully prolong their life.

Incidentally I had to replace the engine starter battery and changed its location to below the port side rear berth. I replaced with a good quality sealed starter battery. The original location get too hot and an AGM is not the best battery for starting. The good news is that this mod can be achieved without altering any of the cabling.
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Ratbasher View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ratbasher Quote  Post ReplyReply Direct Link To This Post Posted: 01 July 2018 at 18:24
While battery monitors are useful, the only definitive check is with a multimeter measuring voltage directly at the terminals. As a guide:

100% - 12.7-13.2v
75% - 12.4v
50% - 12.2v
25% - 12v
Disch/flat - =< 11.8v

If the multimeter confirms you're on 11.9v then ignore the AH reading from the monitor and recharge asap.  You must have a separate engine start battery; after you've been using the domestic bank for a while check that the engine start is showing a different (hopefully higher!) voltage.  If at the same voltage as the house bank then its' not isolated and you might soon be practicing docking under sail.  If it is properly isolated then you needn't be concerned about starting the engine.

I'm afraid it sounds like you might have missed something on setting up the BEP and calling in an electrician is not a bad idea.  Monitors need to be programmed specifically for the house batteries and should not include the engine battery unless it has dual capability.  The readings are curious - perhaps the AH rating of the engine start was included while voltage monitoring was just of the house bank?!?  

Good luck! 



Rat

H400 #311 'Wight Leopard', cruising UK & Ireland

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Ratbasher View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ratbasher Quote  Post ReplyReply Direct Link To This Post Posted: 01 July 2018 at 18:35
Forgot to add that if the multimeter confirms you're down to 11.9v overnight then you've either suddenly developed a bad drain or your batteries are indeed shot.  I'd expect AGMs that have been looked after to last at least 5 years though.  Do check that your charger is set for AGMs and not lead acid as the former demands a lower voltage during the bulk-charge phase.  
Rat

H400 #311 'Wight Leopard', cruising UK & Ireland

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Jeremy View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jeremy Quote  Post ReplyReply Direct Link To This Post Posted: 01 July 2018 at 19:32
Very useful information! My assumption is that the BEP monitor is set up wrong in the house Ah reading and should have been showing around 50%, not 90%. 
Im also assuming that my batteries may be on their way out as I've never struggled to get them through the night without dropping to an alarmingly low level just running the usual items, fridge, lights.

Can anyone else tell me what they would expect to pull in a night? either in V or Ah.
Thanks again.
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Ratbasher View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ratbasher Quote  Post ReplyReply Direct Link To This Post Posted: 02 July 2018 at 09:29
Hi again Jeremy

I recently did an audit of power usage on my own boat.  Unsurprisingly, in the present run of hot weather in the UK the fridge was the biggest culprit running for 2 mins every 20 and drawing just over 6AH when operating.   Evenings alongside, the TV/PC combined usage was about 10AH but the drain from the LED lights was only about 0.1AH each. With the size of your bank at 330AH similar usage should certainly leave a voltage very far above the reading of 11.9 that you're getting now.  

Strongly recommend checking with a multimeter, though.  They cost peanuts but can save you a packet(!)

Interesting to hear the experiences of others.

Rat




Rat

H400 #311 'Wight Leopard', cruising UK & Ireland

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Samtutuki View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Samtutuki Quote  Post ReplyReply Direct Link To This Post Posted: 02 July 2018 at 10:29
It might be worth exploring this type of battery monitor:

http://www.smartgauge.co.uk/products.html

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Samtutuki View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Samtutuki Quote  Post ReplyReply Direct Link To This Post Posted: 02 July 2018 at 10:33
And this is an interesting article too:

http://www.smartgauge.co.uk/sgvahrs.html


Edited by Samtutuki - 02 July 2018 at 10:33
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Touch of Grey View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Touch of Grey Quote  Post ReplyReply Direct Link To This Post Posted: 02 July 2018 at 14:54
Jeremy


I replaced my service batteries (2 * 165 Ah Victron AGMs) like with like a year ago, installed a Balmar SmartGuage (the device Samtutuki mentions) at about the same time and have been monitoring battery performance closely for the last month.


Between 10pm and 7am I am seeing a drop of between 0.15 and 0.25 volts with the SmartGuage saying capacity has dropped between 5% and 15%. During this time, the Isotherm fridge will be drawing 4 amps (according to the manual) or 6 amps (Ratbasher) when the compressor is on,  the Waeco CFX50 fridge will be drawing 8 amps when the compressor is on, and we’ll have at least 4 electronic devices charging.  The inverter which draws 0.25 amps with zero load and the gas solenoid which draws 0.8 amps will both be off. Overnight the temperature has been 15-20C.


If I assume that my total battery capacity is now 300 Ah (a drop of 10% since installing them) and that, on average, I am using 10% of this capacity overnight then that’s 30 Ah over 9 hours or about 3.3 amps per hour.

Allan Fraser
sv Touch of Grey
Hanse 415 / 0102
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Jeremy View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jeremy Quote  Post ReplyReply Direct Link To This Post Posted: 02 July 2018 at 20:16
Thanks for your help guys, all useful and exactly what I was looking for. I put my multi meter on the batteries, did a few tests and they have indeed met a fast demise over the past few months. Im taking them to a battery clinic who claim they can rejuvenate them. Failing that i'll be up for new ones and will probably add 100ah at the same time.
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