Batteries

[MichaelGarratt]

I have two batteries linked via the usual rotary switch. Both batteries seem to be 'cranking' batteries and claim '700A cranking power'. Twice recently both batteries have had low charge (probably because we have spent a long time in the marina this year waiting for decent weather). Battery 1 is reserved for domestic use. When motoring batteries 1 & 2 are switched so that both can be charged by the alternator. The idea was that battery 2 was reserved for starting. However, I suspect that battery 1 is be used quickly as it is a cranking battery and should really be a leisure battery. When both batteries are then switched during motoring, battery 1 drains battery 2 and then it takes a long time to recharge them both. The solution seems to be to replace one battery with a leisure battery which will not drain so quickly.

The alternator is working correctly and the batteries hold charge.

I am considering a 4 step battery charger to avoid the problems of draining the batteries when in port but these are quite expensive and the question then becomes the required output from the charger. I'm not sure of the amp-hour rating of the cranking batteries

Does anyone have any views/experince?

 

[pheran]

Clearly a so-called leisure battery is better suited to the domestic application because of its deep-cycle characteristics. But if current drain from one battery to another ie equalisation, is a problem, consider fitting a blocking diode, or better still, a voltage sensing relay. A good idea irrespective of the battery type.

 

[orizaba]

i am sure you will get alot of sugestions to this query,all i can tell you is how mine is set up,i have a stirling regulator/booster fitted to the alternator charging system,the main charge wire goes from alternater to a post of a heavy duty isolater switch from there a hevy cable goes to positive on my leisure battery bank.a cable also runs from the other post on the switch to the starter battery.the advantage of doing it like this is
1 no voltage drop
2 the leisure batteries will get the most charge
3 you cannot destroy the alternator because its always connected to a battery bank when the engine is running
4 you are able to isolate the start battery from the leisure ones
5if your start battery runs down ,providing you use large enough cables you can link the leisure and start bank together as a kind of jump start system.
the disadvantages are
1 you have to remember to enable switch to charge start battery,if we had the engine on for say1 hr i would charge leisure side for 3/4 hr then link to charge start battery for 1/4 hr,this does depend on alternator size and how flat start battery is
2 you have to rember to isolate start battery when you are not using it.
i have a multi meter wire int system to keep an eye on charge states and also have the leisure batteries fused on the input side.
for marina charging we use a 4 stage mobitronic.
the ideal is 2 alternators a small one for the start battery and a larger one for the leisure side,i had this system on another boat but its all down to available space around engine.
unless you spend alot of money the difference between a start battery and a leisure battery is quite small,a s big a battery bank as you can fit and a good sized alternator plus 4 step regulator is a good way forward,
from what you say it would seem that your alternator is unable to keep up with the usage on your system,i would have expected that with most things on with the engine running your batteries should have remained charged
700 hr cranking power means not alot you should see a label on your battery that says 60ah or 100ah something like that,
that is the true size of the battery.
i'm sure that this will cause a biggish debate but all i can say is, my system is relatively cheap,and the batteries have so far lasted over 5 years,that reminds me i'd better check their water levels

 

[VicS]

[ QUOTE ]
I have two batteries linked via the usual rotary switch

[/ QUOTE ] Unless you have a split charge system either with a diode splitter and battery sensing, or a VSR the way to manage your batteries is to start up using the start battery(obvious!) and to leave just this one selected untill it has recharged. Then switch (via the "both" position to the domestics battery only. Leave that selected on its own until you next want to satrt the engine.

Selecting "both" other than when changing from "engine start" battery to "domestic sevices" battery while the engine is running is inviting trouble as you seem to have found.

Whether or not you opt for a leisure battery or even a true deep cycle battery as the domestic services one is your call but personally I would stick with one that could start the engine if necessary. It seems daft to me to have a second battery that could not be used for that purpose.

Regarding the capcities of your 700 amp batteries I would estimate that it is about 90 Ah but that is based on the 70Ah one in my car being rated at 540amps.

 

[pvb]

[ QUOTE ]
When both batteries are then switched during motoring, battery 1 drains battery 2 and then it takes a long time to recharge them both. The solution seems to be to replace one battery with a leisure battery which will not drain so quickly.

[/ QUOTE ]There's very little drain from one battery to the other when they're both connected to the alternator as you've described. A leisure battery won't "drain so quickly". If you have a problem recharging your batteries when motoring, you need to consider upgrading your system. The cheapest upgrade is to add extra battery capacity to your domestic bank - this will also speed the recharging process. The next upgrade is to add a "smart" regulator (Adverc, Sterling, etc) which will boost the charging voltage when you're motoring and so increase the charge current. Blocking diodes and VSRs really won't make any difference as long as you're using a rotary 1-2-Both switch.

 

[pvb]

[ QUOTE ]
Selecting "both" other than when changing from "engine start" battery to "domestic sevices" battery while the engine is running is inviting trouble as you seem to have found.

[/ QUOTE ]That's not true - the power consumed during engine starting is minimal, and is replaced very quickly. There's no problem leaving the switch on "Both" while the engine's running - the various batteries will only take such current as they can, depending on their state of charge.

My view (repeatedly stated here!) is that 1-2-Both switches are antiquated relics of olden boating and have no place on a boat in the 21st century. They have huge potential for being set in the wrong position, leading to a flat starting battery. The marina I keep my boat in frequently has yachts towed in (sometimes by the lifeboat) after a wrongly-set 1-2-Both switch flattened the start battery.

 

[orizaba]

i would agree over the statement that i-2-both switchs are potentially a problem for another reason,some of them when changing from 1-2 1-both disconnect the charging wire mommentarily and so could cause the alternater to blow as there is nowhere for the current to go

 

[Mudhook]

1. Charging on both is a no-no.

2. Using a non-leisure battery for leisure use is a no-no.

I'm not sure what a 4-step charger is; unless is connected to shore power it won't avoid the problems of draining your batteries when in port. And of course if you have shore power you won't be having these battery problems.

Various "smart" chargers are available, the best known being Adverc and Sterling. What they do is bring the batteries up to full charge more rapidly. They don't force feed them but charge in a way that does less harm to the batteries and gives a better use of their capacity. In conjunction with blocking diodes you get a battery charging system that needs no human intervention; both batteries will be charged at an optimum rate and the most discharged battery will be discharged first. The 1-2-both switch is then simply used to switch to the starting battery to start and then returned to the domestic battery position. A criticism of blocking diodes is that there is a voltage drop across them; however if a slight mod is made to the alternator so that voltage is battery sensed then this drawback is overcome.

I've had an Adverc/blocking diode set up for four years and it's never given me a moment's anxiety about the batteries.

Finally, although some authorities say you shouldn't use a "leisure" battery for starting, a 1-2-both switch gives you the opportunity to use your domestic (i.e. leisure) battery as a backup starter battery. It won't kill it, but repeated use for starting will do it no good. I kept my 1-2-both for emergency backup, but since fitting the Adverc system I've never had an engine problem that the started battery couldn't handle.

Regards, Mudhook

 

[PeterGibbs]

A lot depends on the following:

1. Do you spend quite a lot of time in marinas where you can connect up for charging?

2. Are you going to keep the boat for several seasons longer?

If the answer is yes to both I recommend you go:

3. For an alternater output charging and switching device, as mentioned by others, which will do a much better job of loading your batteries, prolong their useful life, and payback in many ways. They will not revive a half dead battery.
All the switching is automatic - no more hand switches etc.

4. Install a mains battery charger (I have the Merlin, multi stage effect) which will keep your batteries in great shape whilst connected to the shore supply. This means winter too - just connect up and leave for x months - no probs!

This way you can, within reason, largely take your batteries for granted. The above is not cheap - allow £450 for both
plus fitting - hence the question about getting a payback!

PWG

PS. Cranking output is best work output - ie is the battery fit for purpose. Amphere hours is what you should conentrate on - how much work/output is available from the batteries when in peak condition - deep cycling types will normally yield 60% of their plated capacity before voltage starts to drop; at about 10.4 volts most 12 volt sensitive functions will cop out.
A starter battery of 70 Amph is what's found in most boats and will cycyle many times = several years before replacement. Just think of the work such a battery performs in a car!

PWG

 

[jb2006]

Why is charging on both a no-no? - we always used to do that when we had a 1-2-both-off switch. Admittedly, if the leisure battery is very flat there is a slight risk of overheating/exploding the leisure battery by discharging the engine battery through it. The alternator will be putting out more volts than either battery - the rate of charge will be roughly inversely proportional to the voltage remaining and the internal restistance of the batteries. This may limit the amount of charge going to the leisure battery but I dont see any real harm being done. Once you are sure the start battery is topped up it is definitely preferable to divert all the charge to tthe leisure battery

 

[gandy]

Same question here. Why is charging on both a non-no? Or asking for trouble. That's exactly what a VSR does.

 

[VicS]

[ QUOTE ]
Why is charging on both a non-no? Or asking for trouble

[/ QUOTE ] It is not a complete no-no it is just that with batteries parallel there is a risk that a duff battery will grab all the output from the alternator or even run the decent battery flat. A diode system will prevent the latter happening as will a vsr. A vsr will also prevent a duff battery grabbing all the current, at least it will if it's the services battery that is duff but not if the engine start battery fails.

Using a manually switched system both situations can be prevented totally if the batteries are never normally paralleled.

A vsr only charges both together once the starter battery is charged.

It is possible to get a VSR combined with isolating and paralleling swiches. Possibily offering the best of both worlds.