Using Deep Cycle Battery as Car Starting Battery

john p

Re: Using Deep Cycle Battery as Car Starting Battery

Cariboocoot wrote: »

And here's me living in a place with -35C Winter temps starting my vehicle with an ordinary automotive battery bought at Wal*Mart. How silly of me not to realize I needed to spend a lot more money on a deep cycle battery whose design is not suited to operating a starter motor.

Hey boys and girls, you do not need a deep cycle to start your car. You just need a good battery. Can you start it with a deep cycle? As I've already said, yes you can.

Now if you try starting your big truck diesel with a SLI (Starting Lighting Ignition) battery meant for an econobox it isn't going to work well. Likewise if you try starting your econobox at -35C with an old battery it isn't going to work well. You can start either with a deep cycle or an SLI providing it has sufficient capacity. That's all there is to it.

The deep cycle will NOT start the vehicle any better than a proper automotive type battery and for most vehicles the deep cycle will not be charged properly by the alternator. Buying a deep cycle for your vehicle is a waste of money. Just get the right battery for the job.

If your engine is struggling to turn over then the battery is inadequate and/or you've got wiring problems. Fix the thing properly and this doesn't happen.

It never ceases to amaze me how many people try to make a piece of equipment do a job it was never designed for, and then complain about how poorly it works in the incorrect application. In this case we have people singing the praises of the wrong battery for the job just because it happens to be able to do it.

The world would be a better place in battery land if they understood batteries as well as you do..

As you so rightfully said to some of the posters just because they have been getting away with using the wrong battery/ies it does not make it right.... You can put screws into wood with a hammer and in many cases that will work but a screwdriver is the best choice for best results consistently.

007trains

Re: Using Deep Cycle Battery as Car Starting Battery

I get that a new car battery is better than trying to use a deep cycle battery but but i was wondering if a different configuration would work.
I have a bank of supercapacitors that i've been using as a replacement car battery which has worked well (started successfully with just 8volts in it).
I also have a second-hand deep cycle battery.
The current battery i'm using is on its way out and needs jumpstarting if i leave the car for more than a few days
I was wondering if i hooked the supercapacitor bank and deep cycle battery in parallel with each other how well would they work for a car starter battery?
Would it still work if i wanted to run a fridge or inverter off the car overnight?
I'd like to give this setup a try just to see what happens and before i do i'd like to see what you guys think

Cariboocoot

Re: Using Deep Cycle Battery as Car Starting Battery

Welcome to the forum.

Gee, you must be rich to be able to afford super capacitors instead of a car battery.
Capacitors produce NO power; they only store and release it.
In theory your plan to parallel the caps with a deep cycle works: the caps can store up power from the deep cycle over time but release that power quickly when needed for starting. But it is still 'borrowed power' unlike a proper SLI which can do the same thing without extra wiring and at a fraction of the cost.

As for running a 'frige over night and starting the car in the morning, there are a number of factors involved not the least of which is how much power the 'frige uses. To that the caps will contribute nothing, really. Will it start the car in the morning? Good question. The only reason it is doing so now is because the caps alone manage to retain enough power since the last alternator charge to do so. If you were to let them sit a month they probably would not (unknowns: size of caps and demand of starter). With the overnight discharge added in would there be enough power in the morning? Again we have the unknowns of starting power demand and deep cycle capacity.

On the whole it's a pretty dicey for being able to get to work in the morning.

007trains

Re: Using Deep Cycle Battery as Car Starting Battery

The capacitors i got second-hand pretty cheap and by themselves can turn over the car for a good minute.
As for the deep-cycle battery i know by itself it can run the fridge overnight easily enough.
When they are placed in parallel in theory the capacitors will be doing 90% of the work during starting.
And I don't drive daily so i'm not too worried about risk of not starting i just wanna know if its worth a try seeing as i already have the parts i need.

Cariboocoot

Re: Using Deep Cycle Battery as Car Starting Battery

Two rhetorical questions:

What have you got to lose?
What have you got to gain?

That's about all to making the decision to try based on already having the materials at hand.

BB.

Re: Using Deep Cycle Battery as Car Starting Battery

Be very careful with "true" super caps... They usually only have a few year life and are very sensitive to over voltage. Also, a dead short with a super cap can be hugely more dangerous than with a lead acid battery (which can be scary enough).

john p wrote: »

Ultra caps make great expensive toys but for supplementing a battery bank a total waste of money.
Maxwell caps are good. We have a bank of them at work with a total 100,000 farads and 24v. Just the best toy available. they are mounted on a pallet then a trolly and out the front of the trolly is a nice 3/4 inch copper rod. Now if you wheel this up to a engine block and the tip of the rod gets about 1" from it then the discharge will take large pieces off casing off the block. al in one second.

The main problem with caps is the dramatic drop in voltage as they discharge. ie a 12v lead acid battery full about 14v half full.orhalf empty?about 12v only 2v difference. Now 12v bank caps full 14v half full/empty about 8v. Useless .

If you have excess power that's "wasted" get another few batteries. or bigger ones than you have.. far cheaper and will last longer.
super caps if heavy charged and discharged every day will last you less than a year.. Our first bank of them at worl lasted less than 6 months at a replacement cost of over $120,000 . and the new ones are loosing capacity after 4 months now after heavy use..

And the actual amount of stored power (relative to a lead acid) is relatively low:

BB. wrote: »

Super/Ultra Capacitors are not worth it for the average (and even not so average) application... For example, on your first link:

Ultra-capacitors 48.6V 165F @ $1,800 each. And assume that you run it between 38 volts to 48 volts:

• E-useable = 1/2 * 165 Farad * ((48v)² - (38v)²) = 70,950 Watt*Seconds
• 70,950 WS * 1/60 sec per min * 1/60 min per hour = 19.7 Watt*Hours of storage
• 19.7 WH / 48 volt bank = 0.4 AH @ 48 volt battery equivalent

I may have gotten something wrong--But I don't think so... A capacitor of this size just does not compare to very well to a typical storage battery.

You can get more useful energy out of the capacitor by draining it further--But that would mean more issues for the energy conversion device (operating over a wider input range, higher currents required at lower capacitor voltages, etc.).

-Bill

Looking at your numbers... Say you can crank an engine for ~1 minute at ~100 amps:

100 amps * 1 minute * 1/60 minutes per hour = 1.67 Amp*Hour (@ 12 volt) "useful" stored energy

-Bill

Cariboocoot

Re: Using Deep Cycle Battery as Car Starting Battery

BB. wrote: »

Looking at your numbers... Say you can crank an engine for ~1 minute at ~100 amps:
-Bill

That means you need your car fixed. One minute is a pretty long time to be cranking an engine over.

007trains

Re: Using Deep Cycle Battery as Car Starting Battery

Car usually starts within a few seconds the one minute figure was with spark plugs disconnected.
I'm give it a go but how well will an alternator charge a deep cycle battery?

ILFE

Re: Using Deep Cycle Battery as Car Starting Battery

007trains wrote: »

I'm give it a go but how well will an alternator charge a deep cycle battery?

I wrote in this post on Page 1 of this thread, some information about my use of deep cycles in a vehicle.

Anyway, I ran a high output (200 ampere rated) alternator and two Optima Yellow Tops in a 1990 Jeep YJ for several years. It never had a problem charging them.

I recall one incident upon arriving home one night. I left the headlamps on and totally depleted one battery. The next morning, when I went to start it, obviously nothing happened. I changed from one battery to another, via the dual battery switch. I started the Jeep, switched back to the dead battery and went on my way.

I'm sure I did that more than once during the time I had those Yellow Tops in my Jeep. But, they always recharged fully, and neither ever gave me a bit of trouble, up to the time I sold the ol' girl.

007trains

Re: Using Deep Cycle Battery as Car Starting Battery

Thanks.
I currently have an 80amp alternator and i know it won't be as fast as a 200amp alternator but will it be ok for now i.e. the alternator won't be damaged by charging the deep cycle battery

BB.

Re: Using Deep Cycle Battery as Car Starting Battery

Deep cycle batteries (flooded cell) need higher voltages to quickly and fully recharge after deep cycling (and higher voltage for equalization.

I believe the Optima are all AGM type batteries, which live just fine at ~14.0-14.2 that car alternators output just fine.

Automotive alternators tend to run hot if you pull rated current from them, and as I understand, tend to reduce their current output.

Balmar Marine makes alternators designed for use with deep cycle batteries and rated current output--So they are a nice choice for alternative to the simple car alternator (if you can fit them in the application).

Welcome to Balmar

This thread (click on PNjunction) and this post have some more information on the Optima and other batteries:

PNjunction wrote: »

heh, even Odyssey specs a litle different at C10..

What I found maddening when I started out with TPPL AGM batteries - Optimas, Odysseys, and Hawkers, was the profound difference in loose specs for the Optimas, and tight specs for the Odysseys / Hawkers. They come from the same family lineage back to Gates rubber, and searching for ancient docs was quite revealing especially when lead-acid EV was the rage. I suppose the difference was due to the cultural difference between the two initial sales demographics, automotive consumers with Optima, and commercial / military UPS type customers with Odyssey.

It is clear that they are not specifically designed for RE use, and for me, putting them into solar cyclic operations means that to keep them healthy, I have to adopt a bit of a different tactic - namely occasionally hitting them HARD, (something like 0.4C or higher) as long as you are properly voltage-regulated! That will get them up to 90% or so of SOC due to Peukert. They STILL need a slower current charge with a longer absorb, and longer float for internal cell balancing - something I can do easily with my typical 0.2C solar setup.

Search for a pdf from NREL / DOE and using Optimas with high-charge currents to extend life entitled:
"Charging Algorithms for Increasing Lead Acid Battery Cycle Life for Electric Vehicles"

What we do have with solar, is the ability to precisely set our voltages, and can thus run higher recharge currents if we have the panel capability - especially helpful if you are in an area of low solar-insolation, something one may not be able to do with standard agm's, even high quality Trojan agm's accept no more than 0.2C. This is similar to the more common consumer agm's like Powersonic, Universal Battery, etc.

Some may find this non-commercial (NO SALES) site of interest:
http://www.wheelchairdriver.com/fast-charge-your-power-wheelchair.htm

This guy lives on batteries as a paraplegic engineer. TPPL Optimas to Odysseys and now to Lifepo4 to power his custom chairs.

This helped me get over my own initial fright of charging Optimas and Odysseys with a lot of current. As long as one is safe about it, no thermal runaway drama was experienced here - and that is even during 90F ambient temps. Of course I am temp compensated and using batts from a reputable dealer - no used / abused / recycled batts passed off as new, mishandled, stored for a year in the desert in a tin shed ....

If one needs super fast solar recharging, then also look into Lifeline as well. In the end I'm brand-agnostic, and found that while not intended specifically for RE, Optimas and other TPPL's are ok to use with solar, but for the longest life, need some higher-current preventative maintenance charging once in awhile.

-Bill

Cariboocoot

Re: Using Deep Cycle Battery as Car Starting Battery

007trains wrote: »

Car usually starts within a few seconds the one minute figure was with spark plugs disconnected.
I'm give it a go but how well will an alternator charge a deep cycle battery?

What deep cycle battery is the big issue. Also how deeply it will be discharged. Without knowing its capacity and DOD it's hard to predict how much power it will need.

12 Volt 100 Amp hour deep cycle, for example, used for starting in a few seconds won't be much different from a regular SLI, except for being more expensive and able to be discharged continuously down to 50% and brought up again repeatedly.

PNjunction

Re: Using Deep Cycle Battery as Car Starting Battery

Wow - didn't realize how much I rambled on...

SLI vs DC - you may want to consider the application. Obviously if you have a persistent power draw external to vehicle starting, then a 20% or so increased capacity DC battery would be wise.

You may also want to use an increased capacity DC battery in cases where the vehicle does not get started frequently. The damage done to a DC battery by ignoring it is less than the damage done to ignoring an SLI. Not that ignoring battery maintenance is a good thing!

For example, even though Optima Red Tops and Blue/Yellow tops are agm's with low self discharge, the rate of sulfation and deep discharge recovery is more successful with the DC Blue/Yellow's after an accidental deep discharge, or long(ish) periods of neglect. Shallow-cycling an SLI like the red-tops from say 50-70% DOD due to infrequent charging does more harm to them than would a DC Yellow/Blue. Not that doing this to a DC without a full recharge is a good thing either.

Obviously proper maintenance beats having to make a choice based on abuse of either type.

TheLandRover

Bumping this old thread in order to keep conversation in one place....

I've read every post in this thread and while I appreciate the theoretical debate my needs are more practical. I really am looking for a battery that can both serve as a car starter and that can be deep cycled frequently (as in 300 times a year). For reasons that aren't worthwhile, a dual battery setup simply isn't practical for me. Whether the battery is a "true deep cycle" is not relevant to me. I just need a battery that I can (1) deplete to 50% and when it is depleted to 50% it will (2) start my car again (occasionally but not normally in sub-freezing temperatures) and that can be (3) recharged by my car's alternator and not (4) destroy any electronics in the process. All I want the battery for is to power a laptop computer (via an inverter).

Does such a thing actually exist and if so does anyone have any specific recommendations for a particular battery?

BB.

In theory, if you stay in the Lead Acid battery family, a Deep cycle battery should be 20% larger (AH rating) than the standard car battery.

Can you plug in an AC charger at night to "fully" recharge the battery (at least on a weekend day)?

If you get a flooded cell battery (one that you can open the caps on), you should get a hydrometer and see how the battery performs with the stock vehicle charging system and your daily loads... If the battery never does get "fully charged" (say >90% state of charge once per week, or falls more than 50% SOC in average use), then you need to look at "improving" the charging system somehow.

-Bill

PNjunction

TheLandRover wrote: »

I just need a battery that I can (1) deplete to 50% and when it is depleted to 50% it will (2) start my car again (occasionally but not normally in sub-freezing temperatures) and that can be (3) recharged by my car's alternator and not (4) destroy any electronics in the process. All I want the battery for is to power a laptop computer (via an inverter).

Starting at a planned 50% DOD reliably over time may be a risk, no matter what you get, be it an Optima or a really nice Odyssey. Part of that problem is that you would have to drive a LOT to really get the batteries fully charged again, but in the real world you'd be running in a deficit-charge scanario, quickly killing those expensive batteries.

I think the most practical solution here is to get additional laptop batteries, and use a PSW, or pure-sine-wave inverter to power the laptop (and it's own internal charger!) cleanly while mobile. Or use a dc-power supply accessory if your maker has that option.

What many forget when using an MSW or modified-square-wave inverter with a laptop, is that it might seem to work ok, but quite frequently the battery charger doesn't work well on the laptop battery itself!. Go PSW to power laptops with internal chargers for sure.

The costs may be the same, but the simplicity and reliability of just having more laptop batteries on hand that can be charged via the psw inverter means you'll make it out of the driveway a few months down the road.

TheLandRover

[USER="12207"]2004tims[/USER]

PNjunction wrote: »

I think the most practical solution here is to get additional laptop batteries, and use a PSW, or pure-sine-wave inverter to power the laptop (and it's own internal charger!) cleanly while mobile. Or use a dc-power supply accessory if your maker has that option.

That is what I am doing now. The problem is that I do a lot of video editing, which kills my batteries quickly. I have four laptop batteries and I used them all up in a single weekend. Moreover, laptop batteries are not cheap--the price of the four I have right now is ~$200, which I think would net me a decent deep cycle battery and I think a deep cycle battery is going to last a lot longer than my laptop batteries--which usually begin to fade after about 100 recharge cycles. There is also the issue of hassle--regularly swapping out laptop batteries, keeping track of which is charged and which is not, etc.

Really more than anything else what I want from a single deep cycle/starter battery is a way to reduce my cognitive load and so to focus on my work rather than battery tending.

PNjunction

In the end I think you'll just have to pick out a compromise in convenience since a dual-battery setup is out.

How about a dedicated deep cycle that you use for the shoot that gets tucked away somewhere, and when you get back home you charge up? No more swapping out of laptop batteries at least and will let you concentrate. The inverter should have a low voltage disconnect in case you forget and will help prevent a catastrophic total discharge.

I'm thinking along the lines of the biggest AGM Trojan, Lifeline, Odyssey, or Optima (blue or yellow, light case). Not true deep cycle, but at 50% DOD, all of these are along the lines of 800-1200 cycles. You just charge it when you get home asap.

Perhaps actually measure your power draw over time by using a P3-International Kill-A-Watt meter. Measure watts over the hours you intend to use the editing gear. If you don't intend to charge drained laptop batteries, but merely have them waiting in reserve, measure *without* the drained laptop battery, as recharging them of course contributes to the overall draw.

This way, you can find out if even this external battery setup is a total joke, or perhaps may allow you to choose one smaller in capacity than originally thought.

How to start with the kill-a-watt: how many watt-hours can I pull from a 100ah battery?

100ah / 2 = 50ah. (the 50% DOD thing)
50 * 12v = 600wh

So with a total measured from the kill-a-watt, would you make it or require MUCH more? Only you know.

This is a totally simplistic calculation that doesn't take into account inverter loss, peukert if you are hammering a small battery etc, but is a step in the right direction.

sub3marathonman

If you want to do some additional research, the people at PriusChat, with member Ingineer one I can think of, but there are others, and also on the Prius_Technical_Stuff@yahoogroups.com with cor_van_de_water the first guy I think of there. The basic conclusion as far as for the Prius has been the Optima Yellow-Top battery. Now, for a typical vehicle that has to start the engine as opposed to just boot the computer, you will definitely want to heed BB's advice about the CCA, although it is difficult to determine for deep cycle batteries as that isn't typically published. Now, if you got a Nissan LEAF, you'd have about 20kw to work with!! A final place might be Richard Factor's PRIUPS, where he shows how he uses a Prius as a uninterruptible power supply.

I'm going to assume it is a Land Rover that you're working with, so it should be possible to get a good sized Optima in there. Another choice for batteries is CSB, which they have some heavy duty batteries in their EVX (I think) line that might work good too. With using four laptop batteries, guessing 100 watts each, that works out to a minimum battery capacity of 800 watts, or 67ah, which is a serious battery. So you will probably need to re-explore having 2 smaller batteries in parallel somehow. I know that diesel trucks at least in the past used to come with two batteries, as I had "bought" a Ford F250 diesel from a dealer, only to go back the next day to finalize everything and seeing a giant empty space when we lifted the hood before I paid. They kept that truck for months because they tried to rip me off. LOL

For an inverter, I went with a Rosewill inverter from Newegg, which IMHO is a decent brand with decent quality. I see though that none of them are in stock, so you'll probably have to find something else or look on eBay. For the Prius, there was a fuse to limit the amperage from the plug, so you'll have to check how much can be safely drawn from the plug. If you're really serious though, you can get it set up directly from the battery.

The only other possible problem I see is possibly overtaxing your alternator if it is routinely recharging a 50% depleted battery, so as others have said, it would be a good idea if you had a battery charger that you could hook up ASAP when arriving home. I use a CTEK 0.8 amp charger (http://ctek.com/lt/en/chargers/XS%200.8) for the Prius, as it routinely sits with the hatchback up overnight while recharging the Hymotion battery.

oil pan 4

They do this all the time over on ecomodder.com.
Some guys have done an alternator delete. They power their car electrical system off a large deep cycle marine/RV battery.
Not turning the alternator gives them about a 10% boost in fuel economy even after adding the heavier battery.

Marcus

HA !!

I must have missed the memo on how you can't run a car on AGM or Deep Cycle.

I have been using 2 x AGM 92ah (184ah total) to start and run my van for the past 8 years.

Most people go for one CCA battery and a separate AH battery to run all appliances, with an isolator switch.

Never made sense to me, as i wanted to have the most battery power to run the appliances

One battery was way too small, i see those people keep having flat batteries.

Anyways, I run the two batteries with no isolator.

I have a solar panel to charge, and have upgraded the alternator.

Never run out of power, never had a flat battery, and i use a lot of juice.

I change them every 4 years out of preventative measures, but they still hold 12.6 or 12.7 at that time.

BB.

Welcome to the forum Marcus,

AGM batteries are fine as the car battery. Just make sure that the alternator does not over charge (typically, 14.4 volt or less). And AGM batteries (generally) have higher surge current than the standard FLA battery--So they are great at starting vehicles.

With deep cycle flooded cell lead acid batteries, running a slightly larger FLA deep cycle battery (20% or larger AH) is suggested because deep cycle batteries do not have quite as high of surge/cranking current as a "car battery".

The reason for battery isolators is that, on occasion, the 'house battery' is discharged to dead (somebody left some lights on/AC inverter running/etc. and "killed" the house battery). If you have a dead house battery on isolation, then you still have a good vehicle battery to start the engine (and are not stranded out in the middle of nowhere).

However, if you keep a good watch on your combined battery bank and don't over discharge it, then yes, there is no direct need for separate batteries.

If using Flooded Cell Lead Acid deep cycle batteries--There is one possible downside in cold climates... To fully charge/equalize a lead acid or lead acid in cold climates, charging voltages in the 15.0 to 16.0+ range are common--And it is possible the vehicle electronics may not like that high of voltage.

Since AGMs are not EQ'ed (or at least not elevated voltage EQ charged normally), they have less of an issue with high charging voltages in sub freezing climates.

That you replace your AGM batteries every 4 years is helpful... They generally do not last much more than 7 years in operation--And if you are in a warm/hot climate, they can last even less (every 10C increase above ~25C, is a reduction in aging life by 1/2).

I am guessing you are out of Singapore(?) Marcus, so sub freezing weather is probably not an issue for you in normal times.

-Bill

softdown

Regulars here will be shocked to find I have some different perspectives:

1) Engines are much smaller on average.

2) Lubricants, incl oil of course, have improved dramatically.

3) Viscosity is much thinner. The standard 10W-40ish has yielded to 5W-20ish.

4) Carburetors have yielded to fuel injection with high pressure fuel pumps.

What used to take seconds on a cold morning now takes fractions of a second. Unless the fuel pump is going out - then dig deep in wallet you must.

Batteries offer a lot more CCA than we need as a result. But there are also higher drains when sitting due to all the computers and micro possessors I think. I also suspect that the typical starter battery is not as good as it used to be. Perhaps the manufacturers thought the same things I just outlined? Nah - they just pumped up profits via lower quality in all likelihood.

If you have a spare deep cycle battery lying around? Sure - works fine. Even better if you use a winch on occasion.

I have driven a million miles and own 10 cars and trucks. Plenty of experience with automobile issues.

Charge a typical battery ~ every month. An AGM less often due to slower self draining. Do not keep a charger on a battery unless you have an exceptional charger. They tend to slowly cook batteries and greatly shorten life.

I probably have 30 batteries around here - not including the main solar batteries. I am considering using a charger every day as standard procedure. I should do exactly that. Short trips do not charge a battery properly.

Marc Kurth

I am a firm believer in constant Float voltage battery maintainers for applications that only get sporadic charging. I own three for my boat, riding mower and forklift. Batteries tend to last a long time for me.

softdown

You have exceptional chargers. I think BB has cooked 1-2 batteries with constant float applications. Think I've done it 1-2 times as well.

"Batteries tend to last a long time for me." Scientific data that is.

BB.

"Cheap" 1 amp float charger--Eventually overcharged and killed a ~12 volt @ 200 AH battery bank (I certainly did not check electrolyte level like I should have). Put the same charger on a dead battery (needed to get van working temporarily--I always replace car batteries that have "gone dead" from over cranking/lights left on/etc.--Too many have died days/weeks later on me), and it overheated and split the plastic "cube" (still works, just opened like a clam).

"Expensive" Battery Minder (brand) Charger/float chargers--Have left them on cars for many years and never had any problems with excessive charging reducing electrolyte levels (on 3 cars/pickups). Several models used (2 amp and 8 amp  with battery type selection models). They have all worked very well.

https://www.amazon.com/BatteryMINDer-Model-2012-Maintainer-Motorcycles/dp/B005EKY1EM

Have tried the Costco Duracell 4 amp charger... Not used if very long (in-laws drove off with charger still connected--Repaired cables, but have not used since)--But seems to work/float well on FLA car battery.

https://www.costco.com/duracell-4-amp-battery-charger-and-maintainer.product.100479359.html

-Bill

softdown

If the Battery Minder costs as much as the battery - I may question the economics.

The Duracell really tops off a battery. Seems to spend a few hours in "float" in order to do this.

I just told a friend that I didn't recommend leaving a charger on his ATV batteries 24/7. He is using $35 chargers - so the jury seems to be out. Been doing it for a year - his battery sounded weak when he cranked the engine though - 900cc engine.

BB.

When you have 80-90 year old in-laws that don't drive much--They need a car to start reliably. So far, been very happy. The other option was a phone call for me to replace a dead battery every couple of years because of excessive drain of the electronics (Lojack, GPS, cell phone, computers, etc.) between trips and the occasional forgetting to close the trunk (trunk light).

For me, kids are moving on, retired from work. And the old van+pickup just are not driven as much anymore--Putting a battery charger on vs having randomly dead batteries--Was worth it for me. So far, using Battery Minders and similar, have not had a "dead battery" on any of the 1/2 dozen vehicles I had to take care of (in-laws, mine, etc.).

The cheaper Duracell battery charger seems to work OK (I don't have the years of experience with it vs the Battery Tenders--But does seem to work as a float charger on the battery 100% of the time).

And it was not worth it for me to go out and put a charger on each vehicle one day a month to keep the batteries "fresh".

-Bill