Such a thing as too big a battery???

[bammylovechops]

Hi,

I have just exchanged my old batteries on my 2005 Sealine S28.
The original batteries were 95ah, the new ones I have put on are 142ah.
The boat has a smart charger on board and takes a charge no problem on shore power as well.
If it is ok to use 142ah batteries, why are they not installed on manufacture as the cost is negliable.
All appears well with the charger/electrical system.

Your thoughts please.

 

[pvb]

Bigger batteries are better.

 

[TQA]

Hi,

I have just exchanged my old batteries on my 2005 Sealine S28.
The original batteries were 95ah, the new ones I have put on are 142ah.
The boat has a smart charger on board and takes a charge no problem on shore power as well.
If it is ok to use 142ah batteries, why are they not installed on manufacture as the cost is negliable.
All appears well with the charger/electrical system.

Your thoughts please.

In theory it is possible to have to much capacity in relation to current demand and recharging capablity.

However it is not the case for you. Your new batteries will be fine. Just make sure the charger regulator is set to the right type of battery. eg Gel / AGM / flooded cell

 

[knuterikt]

Hi,

I have just exchanged my old batteries on my 2005 Sealine S28.
The original batteries were 95ah, the new ones I have put on are 142ah.
The boat has a smart charger on board and takes a charge no problem on shore power as well.
If it is ok to use 142ah batteries, why are they not installed on manufacture as the cost is negliable.
All appears well with the charger/electrical system.

Your thoughts please.

The producer of the boat probably not be able to increase the price with the differnce between the two batteries. So larger batterie = less marging ££.
Every £ they can cut in production cost without lowering the price will give better margins.

Why does some new sailboats come with foresail sheets that are to short to furl the sail completely. Why are the main sail hallyard so short that it is impossible to take it off the headboard an attach it at deck level.
Answer: somone used a calculator 4 m rope * price/m * x boats = ££££

Batteris can be to big - If they never or seldon get fully chatged they will deteriorate fastyer

 

[Plevier]

The charger needs to be big enough relative to the battery - as a rule of thumb at least 10A per 100Ah, better 15-20A per 100Ah PLUS any significant standing load. Subject to that the bigger the better!

 

[VicS]

If you look at many of the Volvo engine owners manuals you will find they specify a maximum battery size !

Presumably there is technical reason for this !

 

[pvb]

If you look at many of the Volvo engine owners manuals you will find they specify a maximum battery size !

Presumably there is technical reason for this !

Whatever the reason, they're wrong and it's completely irrelevant.

 

[VicS]

Whatever the reason, they're wrong and it's completely irrelevant.

I'll leave you to tell them.
Generally its not smart idea to dismiss manufacturers stipulations in this way.

 

[pvb]

I'll leave you to tell them.
Generally its not smart idea to dismiss manufacturers stipulations in this way.

Oh, come on, Vic! Be sensible. I have a Volvo Penta 2003T on my boat, the workshop manual says maximum 70Ah battery! In fact, I have around 700Ah - ten times as much - and it all works. I suspect the Volvo "advice" is a translation error.

 

[elton]

Oh, come on, Vic! Be sensible. I have a Volvo Penta 2003T on my boat, the workshop manual says maximum 70Ah battery! In fact, I have around 700Ah - ten times as much - and it all works. I suspect the Volvo "advice" is a translation error.

It would be interesting to see what would happen to your alternator, rectifier and regulator if you tried to charge them from flat (assuming you could somehow start your engine of course).

 

[VicS]

I suspect the Volvo "advice" is a translation error.

No it appears in too many different manuals for it to be a translation error.

I think it only applies to the starter battery.

I think the reason is is that with an excessively large battery a much larger current could flow under stalled conditions than the starter ( and the wiring) is designed to withstand. Possibly burn out a motor which would otherwise survive. Maybe "set fire" to the wiring in the process.

I only think, I do not know, but if Volvo specify a max battery size there must be a reason.

 

[halcyon]

If it is ok to use 142ah batteries, why are they not installed on manufacture as the cost is negliable.
All appears well with the charger/electrical system.

I've supplied Sealine now for 30 years, and a nominal 95 ah has been the standard battery as long as I remember. It suited the engine range they used, and by using multiples, normally 2 or 3, you had your domestic bank, So only 1 battery needed purchasing, only 1 battery needed stocking, and it fitted all boats.

Only thing to watch is charge cable size, bigger batteries, more charge current can be drawn, also more load on charger and alternator. So check charge current after fitting.

Brian

 

[halcyon]

I think the reason is is that with an excessively large battery a much larger current could flow under stalled conditions than the starter ( and the wiring) is designed to withstand.

Starter motor stall current is not battery related, most batteries within reason will supply it. Most engines including single cylinder will be around 500 amp max, but can be 1000 amp but for less than 1 sec. Longer times can burn out the windings, most starter motors are only rated for 30 seconds use in a 1 hour period in normal operation.

May be related to bhp rating, which is based on a max battery size, bigger battery bank equals more horse power to run the alternator, less shaft horse power.

Brian

 

[William_H]

Battery size

As said an excessively large battery feeding a starter motor through suitably heavy cables may overheat and damage the starter motor. Most starter designs are predicated on about 9 volts being available from the battery at crank start the current then being limited by the starter motor winding resistance. ie at 500 amps is 4500 watts and .018 ohm.
Fit a big battery which holds up 12volts at start of the cranking current will be 666 amps and 8000 watts.
That is power that the windings must dissipate and added current that the brushes must carry. Hence chance of starter damage. Note here that the current will diminish as soon as the engine turns so will not be so bad. Hence many people do start engines with large batteries attached and don't have problems but there is a risk.

The disadvantages of a bigger battery are more weight, more cost and if they are completely flat you must be careful that the high charge current will not damage charger or alternator.
However if batteries are not allowed to discharge completely. (and they should never be allowed to do so.) then larger batteries can only be better. Perhaps more self discharge so a higher maintenance charge might be needed over long term storage. good luck olewill

 

[jfkal]

I tend to believe that it should say minimum and not maximum.
Logically a 70Ah is just about right for a diesel engine. Any smaller would be a problem (winter, pre-glow, not starting right away etc) as a smaller battery would also not provide the MCA needed ) . Bigger does not hurt. Too big may sink the boat and burn a hole in your pocket being the only real dangers I could see.

Oh, come on, Vic! Be sensible. I have a Volvo Penta 2003T on my boat, the workshop manual says maximum 70Ah battery! In fact, I have around 700Ah - ten times as much - and it all works. I suspect the Volvo "advice" is a translation error.

 

[jfkal]

Nothing, as it would just (at best) happily crank out its designed output.

It would be interesting to see what would happen to your alternator, rectifier and regulator if you tried to charge them from flat (assuming you could somehow start your engine of course).

 

[pvb]

No it appears in too many different manuals for it to be a translation error.

I think it only applies to the starter battery.

I think the reason is is that with an excessively large battery a much larger current could flow under stalled conditions than the starter ( and the wiring) is designed to withstand. Possibly burn out a motor which would otherwise survive. Maybe "set fire" to the wiring in the process.

I only think, I do not know, but if Volvo specify a max battery size there must be a reason.

Others have pointed out the flaw in this argument, and anyway if your argument were correct surely Volvo would have quoted a maximum CCA rather than a maximum capacity?

It also raises a question over the use (which you frequently mention here) of a single large battery bank rather than a separate dedicated starter battery.

 

[pvb]

It would be interesting to see what would happen to your alternator, rectifier and regulator if you tried to charge them from flat (assuming you could somehow start your engine of course).

Nothing untoward would happen. The domestic bank is charged by a 90A Prestolite alternator, which will happily run flat out for hours if needed.

 

[VicS]

As said an excessively large battery feeding a starter motor through suitably heavy cables may overheat and damage the starter motor. Most starter designs are predicated on about 9 volts being available from the battery at crank start the current then being limited by the starter motor winding resistance. ie at 500 amps is 4500 watts and .018 ohm.
Fit a big battery which holds up 12volts at start of the cranking current will be 666 amps and 8000 watts.
That is power that the windings must dissipate and added current that the brushes must carry. Hence chance of starter damage

Thanks Will. You saved me a lot of typing!

Actually I have some actual figures for one (810 watt Bosch) starter. Stalled a current of 400 - 490 amps at 7 volts is quoted. If the battery is very large, instead of the 60Ah recommended in this case, and able to maintain nearer 12 volts the current could be around 700 amps. Damage would occur much more quickly!

I tend to believe that it should say minimum and not maximum.

but it doesn't! and it is consistent through out a number of manuals

Others have pointed out the flaw in this argument, and anyway if your argument were correct surely Volvo would have quoted a maximum CCA rather than a maximum capacity?

It also raises a question over the use (which you frequently mention here) of a single large battery bank rather than a separate dedicated starter battery.

William has explained the logic behind my thinking!

CCA may be a more logical spec for a starter battery but its Ah that have until relatively recently been quoted.

I mention the possible use of the main battery bank for all purposes with just a small reserve battery because if I don't our American friends will, not because I agree with the idea.

 

[bammylovechops]

Hi,

Many thanks for the replies so far, just a bit more info, it is a bank of three batteries, the 142ah batteries are for domestic/starting one engine, and have kept the original battery 95ah for starting the other engine, this has been professionally tested and found to be in good condition.

Mark