Batteries: more frequent, shallower cycling vs. less frequent, deeper cycling

[magictime]

To put this in it simplest terms: all else being equal, what's likely to last longer: a battery recharged from 75% to 100% every day, or a battery recharged from 50% to 100% every other day? (Or more realistically perhaps: 70% to 90% vs. 50% to 90%?)

 

I know that the '50% rule' is supposed to be (in part) a rule of thumb about the most economical use of batteries, but I also know that keeping batteries well topped up is generally good for them. And I'm pretty sure I'd get some funny looks if I threw thick blankets over the solar panels every other morning all through summer in order to minimise the number of charge cycles taking place.

Edited October 26, 2019 by magictime

[Alan de Enfield]

1 hour ago, magictime said:

To put this in it simplest terms: all else being equal, what's likely to last longer: a battery recharged from 75% to 100% every day, or a battery recharged from 50% to 100% every other day? (Or more realistically perhaps: 70% to 90% vs. 50% to 90%?)

 

I know that the '50% rule' is supposed to be (in part) a rule of thumb about the most economical use of batteries, but I also know that keeping batteries well topped up is generally good for them. And I'm pretty sure I'd get some funny looks if I threw thick blankets over the solar panels every other morning all through summer in order to minimise the number of charge cycles taking place.

Simply looking at the number of cycles.

 

Say a battery is good for 300 cycles.

 

Discharging from 100% down to 50% is half a cycle so it will last for 600 of 'your cycles'. Charging every 2 days means you will get 1200 days.

 

Discharging from 100% down to 75% is one-third one quarter of a cycle so it will last 900 1200 of 'your cycles' (days)

 

So, 'cycle wise' no difference.

 

HOWEVER

 

Leaving your batteries in a partially discharged state for two days WILL result in a reduction in capacity which will get gradually worse and worse until they have less capacity than you need to last for two days.

 

Only recharging to 90% will exacerbate the situation even more.

 

Personally I would go for recharging every day - use engine / generator / land line to put the bulk in early morning ( 2 hours ?) and then let the solar 'trickle charge' for the rest of the day and get back up to 99.99% SoC

 

Winter is a whole new ball game.

Edited October 26, 2019 by Alan de Enfield
Edit to correct mathematical errors.

[cuthound]

Batteries are usually specified to a given discharge, for example better quality ones such as Trojans are rated down to 20%. Cheaper ones to 50%.

 

However a complete cycle refer to a discharge from 100% to 0% or the equivalent. So taking the battery from 100% to 75% 100 times is the same as taking the same battery from 100% to 50% 50 times.

 

Providing you dont regularly discharge below 50%, then frequent fully recharging is more important than depth of discharge for longevity.

 

Damn I see AdE got there first. 

Edited October 26, 2019 by cuthound
To add the last sentence.

[Momac]

If on shore power is it best to leave the charger on all the time to keep the batteries fully charged?

Or is it better to fully charge the batteries then switch the charger off and  let the batteries  rest under no electrical load during periods when the boat is unused?

Edited October 26, 2019 by MartynG

[MoominPapa]

23 minutes ago, MartynG said:

If on shore power is it best to leave the charger on all the time to keep the batteries fully charged?

Or is it better to fully charge the batteries then switch the charger off and  let the batteries  rest under no electrical load during periods when the boat is unused?

The former.

 

MP.

Edited October 26, 2019 by MoominPapa
Sense

[cuthound]

I keep my charger on when on a shoreline, as did the previous owner of my boat. 

 

I also try to charge back to 100% every day when the boat is being used.

 

My batteries (Lifeline AGM's) are now 12 years old but still have a round 50% of their original calacity left.

 

However, batteries left on continuous float charge often show a little reluctance to meet high loads for a short while being taken off float, but this soon recovers with use.

[WotEver]

1 hour ago, Alan de Enfield said:

Discharging from 100% down to 75% is one-third of a cycle

Maths not your strong point, Alan?

1 hour ago, cuthound said:

Damn I see AdE got there first. 

But got his percentages mixed up. Yours were correct. 

[Alan de Enfield]

2 minutes ago, WotEver said:

Maths not your strong point, Alan?

But got his percentages mixed up. Yours were correct. 

Oooops !!!

 

Thank you,

[magictime]

OK, thanks all. Still a bit confused as I've seen battery spec sheets with graphs plotting number of cycles against depth of discharge, and these don't tend to have the neat mathematical relationship implied above - e.g. a battery always cycled from 100% to 75% and back doesn't seem to have twice the 'cycle life' of one cycled from 100% to 50% and back.

 

Which makes sense of the rule of thumb that discharging to 50% tends to be most economical, although I guess that only holds if each cycle takes place over the same period of time. Is this really about battery bank sizing - i.e.the idea is, it's more economical to buy a bank where your use between recharges will take you down to 50%, than a bank where it will take you down to 75%, because the latter bank will cost twice as much but won't last twice as long?

1 hour ago, cuthound said:

My batteries (Lifeline AGM's) are now 12 years old but still have a round 50% of their original calacity left.

I wonder how impressive that actually is, though. I mean, isn't 'end of life' usually defined as the point where a battery's capacity is down to 80% of the original figure? In which case yours might have hit it after maybe four years. 

[cuthound]

1 minute ago, magictime said:

I wonder how impressive that actually is, though. I mean, isn't 'end of life' usually defined as the point where a battery's capacity is down to 80% of the original figure? In which case yours might have hit it after maybe four years. 

 

When I was working  in the critical power industry, batteries were changed at 80% of badged capacity and specified so that at end of life the capacity was what was actually required (usually to support design load for a period of one hour).

 

With a boat it is a bit different. My leisure battery capacity is specified at 660Ah, but the present capacity of around 300Ah is more than enough for my needs, as I usually move every day, so only need a battery capable of supplying a few lights and the TV and keeping the fridge on overnight.

 

I will change the batteries when they can no longer achieve this.

[Sea Dog]

3 minutes ago, magictime said:

OK, thanks all. Still a bit confused..

Don't be confused - recharge them immediately after discharge, every day if you can, for optimum life. Personally, with my sealed lead acid batteries, I aim to keep my minimum state of charge above 60%.

[magictime]

8 minutes ago, magictime said:

 isn't 'end of life' usually defined as the point where a battery's capacity is down to 80% of the original figure?

...which I guess fundamentally changes the way the '50% rule' works for practical purposes, too...

 

I mean, if you choose a 300Ah bank because that size fits with your usage and a 100%-50%-100% cycling regime, it probably will be at the end of its useful life when its capacity drops to 80% of badged. But a 400Ah bank, with a regime of 100%-60%-100%, would still have 320Ah of capacity when it hit that point and could afford to drop another 20% and more before you needed to replace it. Plus presumably it would take longer to hit 80% in the first place due to the lower depth of discharge. Which makes the 50% rule look pretty wide of the mark, surely? I still feel like I'm missing something here...

9 minutes ago, cuthound said:

 

When I was working  in the critical power industry, batteries were changed at 80% of badged capacity and specified so that at end of life the capacity was what was actually required (usually to support design load for a period of one hour).

 

With a boat it is a bit different. My leisure battery capacity is specified at 660Ah, but the present capacity of around 300Ah is more than enough for my needs, as I usually move every day, so only need a battery capable of supplying a few lights and the TV and keeping the fridge on overnight.

 

I will change the batteries when they can no longer achieve this.

...or maybe not; I think this illustrates the point nicely!

 

8 minutes ago, Sea Dog said:

Don't be confused - recharge them immediately after discharge, every day if you can, for optimum life. Personally, with my sealed lead acid batteries, I aim to keep my minimum state of charge above 60%.

Yeah, I mean that's been my understanding, but I was reminded of the rule of thumb about 50% discharge = most economical (on the Smartgauge site, specifically) and am trying to work out how this fits in.

[cuthound]

2 minutes ago, magictime said:

...which I guess fundamentally changes the way the '50% rule' works for practical purposes, too...

 

I mean, if you choose a 300Ah bank because that size fits with your usage and a 100%-50%-100% cycling regime, it probably will be at the end of its useful life when its capacity drops to 80% of badged. But a 400Ah bank, with a regime of 100%-60%-100%, would still have 320Ah of capacity when it hit that point and could afford to drop another 20% and more before you needed to replace it. Plus presumably it would take longer to hit 80% in the first place due to the lower depth of discharge. Which makes the 50% rule look pretty wide of the mark, surely? I still feel like I'm missing something here...

...or maybe not; I think this illustrates the point nicely!

 

Yeah, I mean that's been my understanding, but I was reminded of the rule of thumb about 50% discharge = most economical (on the Smartgauge site, specifically) and am trying to work out how this fits in.

 

Yes, the 50% "rule" is a rule of thumb, where it maximises  battery life (discharging leisure batteries below 50% significantly shortens their lives) whilst minimising purchase costs, by not buying a battery massively larger than you need. (I'm the second owner of my boat, but wouldn't have specified such a large battery, or indeed such an expensive manufacturer, had I been the original purchaser).

 

Battery end of life on a boat is better determined by when it no longer meets your needs, rather than when its capacity has dropped by a given percentage.

[Sea Dog]

57 minutes ago, magictime said:

Yeah, I mean that's been my understanding, but I was reminded of the rule of thumb about 50% discharge = most economical (on the Smartgauge site, specifically) and am trying to work out how this fits in.

Really though, who discharges to 60%, 50% or whatever and then recharges?  I just set myself a lower limit and check I'm staying within it.  If I can't, I either need a bigger bank or to moderate my consumption. Similarly perhaps, if you're only taking 10% out each evening in normal use, maybe your bank is too big for your usage pattern and you'd be better saving money and space by choosing a smaller capacity bank at the next replacement.

[MtB]

Point of Order M'Lud....

 

Do you not have a set of lead-carbon batteries now?

 

If so, applying LA rules to LC batts is unlikely to end well....

 

 

 

[magictime]

I do, but (1) lead carbons are a type of lead acid and (2) it was partly just a general question raised by reading around, rather that specifically about my situation. 

[Sea Dog]

10 minutes ago, Mike the Boilerman said:

Point of Order M'Lud....

 

Do you not have a set of lead-carbon batteries now?

 

If so, applying LA rules to LC batts is unlikely to end well....

 

 

 

Absolutely agree - my earlier post did say my sealed lead acid, and that's what I was expanding upon. 

 

I was wondering if the latest lead carbon might be the way ahead, but haven't pulled the trigger yet as I've yet to be convinced. Perhaps I will be by the time my current bank needs replacing?

[magictime]

27 minutes ago, Mike the Boilerman said:

If so, applying LA rules to LC batts is unlikely to end well....

Anyway, as long as I was treating them as the AGM lead acid batteries they are, I don't think any harm would come to them. As I understand it, it's not that they wouldn't benefit from not being discharged below 50% and from being fully charged regularly, like any other lead acid battery; it's just that they should charge a bit faster and not be so prone to losing capacity due to sulphation if in a partial state of charge much of the time.

 

In practical terms, I'm hoping I can charge them daily for 60-90 minutes (while running the engine for hot water anyway), and not stress about it if they only get to 80% or 85% most days and to 100% at least once a fortnight (while cruising). This is winter I'm talking about; it's a non-issue in summer with solar and frequent cruising.

[MtB]

11 minutes ago, magictime said:

Anyway, as long as I was treating them as the AGM lead acid batteries they are,

 

But ARE they?

 

There seem to be so many fundamental differences in their behaviour, I don't think it makes any sense at all to apply received LA wisdom to LC batteries. 

 

But I don't know really, and nor I suggest, do you or anyone else on this forum! 

 

 

[magictime]

38 minutes ago, Mike the Boilerman said:

 

But ARE they?

 

There seem to be so many fundamental differences in their behaviour, I don't think it makes any sense at all to apply received LA wisdom to LC batteries. 

 

But I don't know really, and nor I suggest, do you or anyone else on this forum! 

Well, a wise man knows that he knows nothing. Still, I don't think I'll be led too far astray by 'received LA wisdom' if I connect them up like any other pair of 12 volt AGM batteries, set my A2B and charger to 'AGM' as per Leoch's instructions, try to keep them above 50% or 60% SoC by charging them daily, and make sure they get a full charge at least every week or two. Beyond that there is certainly plenty to learn about their real-world performance, and I'll be curious to find out.

 

I'll admit I'm a bit worried about setting the Smartgauge's battery type, though.

Edited October 26, 2019 by magictime

[WotEver]

3 minutes ago, magictime said:

I'll admit I'm a bit worried about setting the Smartgauge's battery type, though.

Ask Merlin

 

(They’ll ask Gibbo and he’ll say something rude then they’ll respond to you with a polite translation...)

[magictime]

7 minutes ago, WotEver said:

Ask Merlin

 

(They’ll ask Gibbo and he’ll say something rude then they’ll respond to you with a polite translation...)

 I have done; just waiting to hear. Fingers crossed there's something suitable. There's a 'carbon fibre' type setting, so maybe that.

[WotEver]

13 minutes ago, magictime said:

I don't think I'll be led too far astray by 'received LA wisdom' if I connect them up like any other pair of 12 volt AGM batteries, set my A2B and charger to 'AGM' as per Leoch's instructions, try to keep them above 50% or 60% SoC by charging them daily, and make sure they get a full charge at least every week or two. Beyond that there is certainly plenty to learn about their real-world performance, and I'll be curious to find out.

Agreed. Their charge acceptance should be better than AGM but the voltages are similar so I think you’ll be fine. 

1 minute ago, magictime said:

 I have done; just waiting to hear. Fingers crossed there's something suitable. There's a 'carbon fibre' type setting, so maybe that.

As you go through a few cycles the SmartGauge will ‘learn’ their characteristics anyway

[MtB]

14 minutes ago, magictime said:

Well, a wise man knows that he knows nothing. Still, I don't think I'll be led too far astray by 'received LA wisdom' if I connect them up like any other pair of 12 volt AGM batteries, set my A2B and charger to 'AGM' as per Leoch's instructions, try to keep them above 50% or 60% SoC by charging them daily, and make sure they get a full charge at least every week or two. Beyond that there is certainly plenty to learn about their real-world performance, and I'll be curious to find out.

 

I'll admit I'm a bit worried about setting the Smartgauge's battery type, though.

 

But three of the major advantages of LC chemistry AIUI are that you can discharge them down to 20% with no ill effect, you can leave them part charged with no risk of sulphation, and you don't need to charge them to 100% once a week. 

 

Almost as different from LA as lithiums.

 

 

[WotEver]

6 minutes ago, Mike the Boilerman said:

 

But three of the major advantages of LC chemistry AIUI are that you can discharge them down to 20% with no ill effect, you can leave them part charged with no risk of sulphation, and you don't need to charge them to 100% once a week. 

 

Almost as different from LA as lithiums.

But the huge difference between LC and  LI-ion, and why an AGM setting is likely to be correct, is that the charge voltages are almost identical to AGM requirements. Unlike Lithiums you don’t have to stop charging at any point and unlike Lithiums you can float charge them. They also suffer from a more significant voltage droop than ‘normal’ LA which again is very different to lithium. In fact the only two major similarities to lithium is that they don’t sulphate and they will accept a faster charge rate, neither of which will affect charger or monitor settings.