Battery Charging and bridge rectifiers

[Rob Johnstone]

Vagabond sport two 40Ah batteries and there have been linked in parallel both for charging and supplying power to the boat. This winter they have been sitting in my garage and I have discovered that the charge / discharge characteristics of the batteries differ one to the other, so charging / discharging them in parallel is probably not a good idea.

There's been some discussion on this forum before of using a bridge rectifier to isolate the batteries from one another, so that the battery that has less capacity doesn't keep being recharged by the "better" one.

So I have now fitted a bridge rectifier between the batteries and the positive power rail on the boat and between the charging system and the batteries.

Fine. The batteries are now independent and don't discharge from one to the other. The snag is that the wonder battery charger that I have (CTEK) clearly needs to sense the voltage of the battery that it is charging to make it's "on board" charge controller work and the bridge rectifier is inhibiting that and I suspect that the solar panel charge controller will have the same problem.

Back to the drawing board.....

[Graham W]

Rob,

I may be whistling through my nether regions but aren't there solar controllers that can handle charging two batteries independently of one another? This one for example http://www.amazon.co.uk/battery-controller-regulator-motorhome-batteries/dp/B006OA8LKQ

There's also something called a Ctek Dual which appears to be designed to cope with dual batteries being charged from multiple sources - see http://www.ctekbatterychargers.com.au/dual-battery-systems.html

Possibly not so much back to the drawing board as back to the cheque book?

[David Hudson]

Morningstar make a controller that allows independent charging for engine and domestic battery banks. This sounds similar to what you require.

[Rob Johnstone]

Thanks guys. I know there'd be an answer somewhere and Graham is right - it's the chequebook, not the drawing board. It's clearly time I realised that technology has left me not high and dry but in it's wake.

[Peter Taylor]

Rob,

One solution would be to buy a solar charger capable of handling 2 batteries. For charging from the mains the simple solution is just to connect your Ctek charger directly to the terminals of each via a switch so you just charge one battery or the other. Obviously that rules out long term battery maintenance (rather than recharging) but the solar system should handle that for you anyway.

I can't work out whether the  Ctek Dual is an alternative solution which would work with your existing Ctek charger plus your existing solar controller.  If it is, that might be a neat, if expensive, solution.  Possibly it would be as shown in http://www.ctekbatterychargers.com.au/dual-battery-systems.html dual battery system 3.  But I can't figure out what are inputs and outputs on the Dual despite having had a quick look at the manual for it!

Peter

Peter

[Rob Johnstone]

Hi Peter,

Thanks for the idea.
I'm now wondering about a "dual battery" solar controller connected directly to the batteries and connecting each battery to the boat supply through its own diode, thus isolating the batteries from each other (as if they were on separate circuits). I think Ill then resort to crock clips to connect the charger to each battery in turn when required! It's probably the least cost route as I can get a dual battery solar controller for about £50.

In the meantime, I've asked CTEC for their "official" view about whether I could use my existing battery charger in conjunction with their rather swish dual charge dual battery unit Ctek D250S.

Thanks for all the suggestions, I'll let you know the result from CTEK.

[Peter Taylor]

Hi Rob,

Remember that there will be a voltage drop across the diodes probably in the range 0.5V to 1.5V. If you have devices drawing significant currents - e.g. Tiller Pilot, Ballast Pump - it might pay to connect them directly to one of the batteries, particularly if they are voltage critical (the Tiller Pilot needs good voltage to perform at full power). 

When I first got Seatern I had two batteries, not in parallel but each powering different sets of devices (with plugs allowing the roles of the batteries to be swapped or combined if necessary).  Now I have only one battery but still maintain separate supplies to some devices.  Recently I've realised that the ballast pump should have its own independent supply because it is most likely to blow a fuse, e.g. if it freezes up or the impeller jams.  Even though the ballast pump (and everything else)  is fused and switched at the switch board, that didn't stop a main fuse at the battery being blown first, probably due to simultaneous demands from the Tiller Pilot and the Ballast Pump. (More details of the evolution of the wiring on Seatern are described in a 30th October 2014 entry in my blog).

Peter

[Graham W]

Off topic but I wonder how many contributors to the forum when it first started in 2007 would have thought that complicated electrics in Swallow Boats would be much-discussed topics eight years later.  The first real discussion of electrical items came in 2009 with this thread http://www.swallowyachtsassociation.org/smf/index.php/topic,264.0.html

Torqeedo motors made an earlier brief appearance, in 2007, and it's amusing to note that all the shortcomings listed then are exactly the same now.

I wonder what we'll be discussing eight years from now.

[David Hudson]

 Having rewired a 28 footer, a Swallow is joy! I am resisting anything power hungry and one 22ah lithium battery should do the lot!

[Peter Taylor]

  The first real discussion of electrical items came in 2009 with this thread http://www.swallowyachtsassociation.org/smf/index.php/topic,264.0.html

Looking at that thread I guess we'll be discussing exactly the same in eight years time! 

One thing mentioned back then was using a fuel cell.  I have been doing some experimental tests of an Efoy fuel cell, the sort that uses methanol.  To summarise the results: one would fit in a BC20 (a potential site is the forepeak with the exhaust pipe - for very small amounts of water and CO2 - feeding into the ballast tank breather). It would give independence from land power for a period of several days or more (depending on how much methanol you are willing to carry). The mode of operation is to run electrical gear from the normal onboard battery which is then recharged by the fuel cell when the battery has got down to 50% discharge (or whatever you pre-program).  The fuel cell is not used for power directly, for example you can't use it to power an electric outboard. 

The main problem (and it's a very big one) is the cost (order £2500 - £3500 for 40W - 70W units; and they need pure methanol at around £4 per litre which gives about 1 kWh).  What's more they do have a set lifetime (measured in usage hours) so you wouldn't want to use one as your primary source of battery recharging, for example if you boat is kept on a mooring.  For comparison, you can get suitcase type petrol generators which are about the same size, offer more power, and cost less than £1000. However  even the "quiet" petrol ones are noisy if you are sharing a boat with one, and they have a hot and potentially poisonous exhaust.  In comparison the fuel cell is much quieter and much cleaner, it can be quietly humming away during the night recharging the boat battery while you sleep.

If anyone is not put off by the cost and wants more details of the evaluations I've been doing, send me a personal message,
Peter

[Rob Johnstone]

I have now had a reply from the helpful sales team at CTEK

(1) The battery charger that I have  (CTEC 1007) definitely requires feedback from the battery in terms of the voltage that is generated by it's cells (I presume there is some sort of intermittent pulse and sensing sequence from the charger to enable this).

(2) The "dual charger" device produced by CTEK and mentioned in the post above, is really a charge splitter, splitting charging current from two sources (for instance solar panels and a battery charger) and allocating this current between two batteries.
This means that any battery charger used as input to the "dual" charger needs to be pretty unsophisticated - producing a steady charging current at 13.5 V or higher. The complicated stuff is then done by the "dual charger".

So, it's either back to basics and keep the things in parallel or adopt diodes and a "dual battery splitter" to control the solar panel charge and "crock clip battery selection" for the mains charger. I suppose I could adopt a slightly more sophisticated version of this by adding a switch to short out the diodes when the battery volts get low enough for the voltage drop across the diodes to be a problem!

Given my inherent laziness, I think back to basics wins the day!

[Peter Taylor]

Hi Rob,

If "back to basics" means simply connecting 2 lead acid batteries in parallel (or shorting the diodes out) I'd strongly advise against. At best you will end up with a "lowest common denominator" situation having wasted power as one battery tries to charge the other and heats up the connecting wire.  At worst you will have a fire.  A very simple solution is just to use one battery at a time with the selection controlled by a switch (not crock clips which are another potential source of resistance!). You would also need a  solar controller of the dual battery type.

Peter