Electronics maths help!

[Rusty]

I have a trickle charger that recharges the 7ah batteries back up to full capacity. Tested with a multimeter.

It will have a charging rate on it somewhere, either a label on the charger or in the destructions. I reckon you need a 20Ah battery for the new fishfinder to give you some legroom, knowing the rate of your charger will tell you how long it will take to fully charge up the new battery. Your charger may not have the balls to cope with a bigger demand.

Edited August 11, 2015 by Rusty

[chesters1]

I have a trickle charger that recharges the 7ah batteries back up to full capacity. Tested with a multimeter.

 

At 50% capacity it should give a readout of 12.2v, so I would set the battery alarm on the fishfinder to that and then swap batteries if necessary.

 

I have now come up again a load of additional issues, if I get a single battery big enough to run it all day, I'll need a new bigger box to keep it in and will have to reattach the unit head to it. Also the higher batteries have larger terminals, so the connectors I currently have on the power lead would need replacing. Not the worst thing but additional hassle and makes everything bigger and heavier.

 

I'm now leaning more towards getting a couple more 7ah batteries and just switching over when they drop to 50%...

A multimeter only tells you the voltage of the thing you can measure the current by draining the battery using the miliamp part of the meter but its not accurate in determining the capacity of the battery .

You can get a reading of 13 volts with a ten miilamp capacity or 100 milliamps volts is just volts

Is your charger getting enough umph to get past the batteries inbuilt resistance ,put your multimeter across the terminals when its charging the reading should be over 14 volts on a 12v battery ,once you remove the charger the voltage should remain at about 13.8- 13.5 is until you put a load on the battery

Edited August 11, 2015 by chesters1

[Anderoo]

Yes, it's a yuasa 1ah 3 stage trickle charger.

[chesters1]

If its on a boat is it worth the bother of upgrading the battery to a leisure battery ,these are designed to drain far lower than normal batteries.

As you say you will need a bigger box but the leads could be made just to connect to the original female socket and the other end of a fly lead with crock clips or large rings to go on the battery bolts.

Expensive outlay ,new pukka charger needed and the effort of hoiking it about but power enough for everything and they last if their neglected (not left for months uncharged ofcourse)

You can buy over the counter boxes but again expensive.

As for your charger its ok for charging single batteries but for more at the same time (if you buy more batteries of the same kind) not so great .

The modern chargers dont trickle anymore this isnt good for batteries it causes the plates to gunk up so modern charged use delta charging and then sit and monitor the battery ,if the voltage drops it charges it back up.

I have had my camping 160 amp battery on one for 4 years ,it wont be used for camping but i wont throw it away just in case we get a power cut it can power our telly for a few hours LOL

 

Or buy a larger battery box and put a few of your present batteries in it wired up in parallel to give a higher capacity ,every battery you put in the box doubles the capacity but with your present charger you can only charge one at a time so not so good for a quick turn around

 

A 50 amp hr leisure battery and box wont be to big so the size and weight of a single big battery vs several small ones pretty similar but the smart charger you will have to buy will charge car batteries as well so not so much in the long run,just make sure the smart charger (lidl has a decent one now and again i hear) can charge far more than the battery you buy ,it could charge your little batteries in parallel as well if you go down that route

 

I didnt realise it was lead acid batteries i looked at the picture later and went onto dry cell battery mode ,to find out what capacity a wet cell is you can use use a 12v bulb and a meter on amps and measure the time the bulb goes dim vs the time and current.

Not perfect but i expect to get a charger that tells you actual capacity bloody expensive compared to the dry cell battery ones

Edited August 12, 2015 by chesters1

[Anderoo]

I already have a decent charger for car batteries/leaisure batteries (a halfords one) so if I do end up going down that route, that's covered.

 

Yes, these are SLA batteries.

 

Interesting about charging the 7ah batteries in parallel with the bigger charger - how would that work?

[JOHN.H]

Firstly , power is measured in watts . Secondly additional batteries in parallel ( assuming the same battery specification ) will only "double " the capacity for the first extra battery , if every subsequent battery doubled the capacity a lot of the worlds power problems would be solved. Ampere hours are , or can be considered to be a guide i.e. 12 v 10 aH , 1 amp for 10 hours or say 5 amps for 2 hours . Any calculation should also take into account other factors like temperature etc. A factor of 0.7 will take this into account . Incidentally voltage readings should be in parallel and current readings in series . Best to seek expert advice as you will not pass electrical principles reading some of these posts .

[chesters1]

Frying tonight LOL

A device wont know the batteries are in parallel ,submarines and storing energy from 12/24v generators are batteries wired in parallel

Anyway theres hundreds of sites out there from "alternative" energy groups or remote controlled car plane enthusiasts showing you how to connect batteries heres a simple one for starters

http://www.trojanbattery.com/Tech-Support/TechologyLibrary/ConnectionsDiagram.aspx

As i said its not straight forward finding the actual capacity of a sla other than by trial and comparison

My best advice is go for a bigger battery ,less wires less mistakes

Edited August 12, 2015 by chesters1

[Steve Walker]

No point converting amps, voltage and hours to watts and joules in this case, though, because the voltage will cancel out in the final calculation.

 

It's only ever going to be a back of a fag packet calculation, you have to assume that the power consumption of the device is constant, the voltage delivered is constant and the total charge of the battery is constant whatever the discharge rate and ambient temperature. And then throw in a massive fudge factor to allow for not wanting to fully discharge the battery.

[Rusty]

I agree with Chesters, stick to your original plan and get a bigger battery. The hassle and expense of creating a new set up at your convenience is small beer compared to messing about changing batteries in the middle of Rutland when it's bound to be chucking it down. You could always sell on your existing kit as a package with the battery and box.

[Anderoo]

To be honest I don't feel like I need to properly understand the electronics of it all, and fully admit that I have something of an intellectual black spot with anything like this, which is why I asked. I know that if I check a fully charged 12v battery with a multimeter it reads about 13.5v and that when it reads 12.2v it's about 50% discharged and it needs recharging to maintain its lifespan.

 

A single bigger battery is the neatest option and I probably will do that at some point, but in the meantime I'll try the 4 x 7ah batteries in rotation. Swapping them over is a matter of seconds so I'm not worried about that. There is something to be said for keeping the whole lot as small and portable as possible, I don't really like the idea of having a huge box and large battery as standard. A lot of people have one much bigger battery of about 110ah which they use to drive an electric outboard and the fishfinder(s), so maybe I shuld be looking to do that and get an electric outboard...