critical level of battery charge

SteveGorst

SteveGorst

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I have a Simrad TP 20 and last year we sailed to Liverpool from Conwy on an overnight trip. After a few hours the autopilot started playing up but when we switched to nr2 battery it was fine again. I have put this down to the autopilot navlights plotter etc drawing power from the battery to a critical level. I would like to know how to calculate this given that it is an 85ah battery how many ah can i take out of it before the voltage starts to drop?

Thanks
Steve
 
It is generally taken that 0.1 volts is 10% of the capacity (Ah). However, the voltage does not drop linearly; you can consider as a rough working guide that the volts will drop from 12.8v (fully charged) to around 12.6 volts after one hour if the discharge current is at or near the 20 hour rate (e.g. 85Ah at 20 hours = 4A roughly). From 12.62 volts down to 11.8 volts the discharge is more linear and for practical purposes you could consider it linear at least down to 12.2/12.3 volts (the half charge point).
These numbers are a rough guide which I have taken from theory and also from doing load discharge tests on batteries myself.
However, I would suspect that you have more load than 4A with the equipment that you are talking about. In fact the autopilot will probably take something like 1A in stand-by and 3A when under load and you can consider perhaps a 25% duty cycle in moderate wind.
So, what you are probably seeing is the battery patially discharged by the load (lights, auto-pilot e.t.c.) then when the auto-pilot kicks in to correct the heading the current drawn pulls the voltage down even further and crosses the drop-out threshold.

The answer to your question is that the volts start to drop immediately you put any load on the battery, in very simple terms the rate of drop and time taken depend on the current drawn and the capacity of your battery. The solution to this would be to increase the capacity of your battery (ies): smart regulator, bigger batteries, de-suphatisation ...............

Alan.
 
I had this problem with my TP10 when I first got it. It would drop out and go onto standby for no apparent reason. I spoke to Simrad about it and they said it would drop out if the voltage fell below 12v (a bit high in my opinion). Tests on my boat with a digital voltmeter showed that the problem was voltage drop twixt battery & autopilot (I was using old cabling & plug installed yonks ago). Thicker cabling and a new plug cured it.
 
It took a couple of reads but I've got it now. Thanks for the full explanation.

I will have to monitor the voltage at which it starts playing up, at the time I just accepted that it was fixed and didn't get my volt meter out. Maybe I should fit one......

It played up after about 5 hours though which was probably about 50ah, I forgot to mention the coolbox which apparently takes 5amps on it's own!! The voltage would probably have been about 12.2 but dipping lower as the pilot searched for power.

Steve
 
Steve,
If you have an 85Ah battery, then it is likely to be only charged to about 85% (unless you have an Adverc or Sterling e.t.c. alternator regulator); and as you should not discharge a battery much below 50%, then you only really have :
Fully charged = 73 Ah
50% charged = 42 Ah
Available = 31 Ah

So, if you estimate that you had used 50 Ah, I would guesstimate that your battery was at something like 30% or 12.1v; this seems to be near the threshold suggested by someone else.

Alan.
 
If you discharge a battery more than 50% you shorten its life, so an 85 Ah battery will give you about 43Ah - not mcuh with a fridge and an autopilot!

The trouble with measuring voltage to determine the Ahs left in the battery depends on the quality of the voltmeter. A digital one is really the only answer. Also you have to take off the load and wait for an hour or longer to see what voltage the battery returns to. An open circuit voltage of 12.2V is 50% dicharged. Below 11.8% is fully discharged.

Note there is only about .5V between fully charged and 50% charged.

The best way to tell is fit a Battery Condition Meter that measrures amps discharged agains time - and hence tells you the Ah left.
 
[ QUOTE ]
not mcuh with a fridge and an autopilot!

[/ QUOTE ]

No you're right I'll have to find another way of keeping the beer cool!! I will just have to be more careful with use of power as there isn't really any room for bigger batteries without major changes to the battery compartment. The meter you mentioned should help me do that. My wife had been talking about a 12v hairdryer but I thik that is out of the question!! A friend had mentioned the better chargers that Alan mentioned which might be worth getting as they will make my 85ah batteries more effective.

Steve
 
[ QUOTE ]
If you have an 85Ah battery, then it is likely to be only charged to about 85% (unless you have an Adverc or Sterling e.t.c. alternator regulator);

[/ QUOTE ]That is only true if his existing charger is inadequate. Most charging systems these days charge at 14.3V or more, even up to 14.7V without an alternator booster such as an Adverc. Before spending money on the booster and the considerable effort installing it, plus the reduced reliability due to a more complex system, people should check firstly with a meter that their existing charging system is inadequate.

There are hundreds of threads here discussing correct battery charging levels, try a search (if that still works?)
 
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