The 16 year old Interstate MT-101 Flooded Lead Acid battery installed in my Cadillac STS 4 was faltering in the face of a vicious Canadian winter that has already reached daytime lows of -18C in this particular desolate icy hellscape.I will first give Interstate due congratulations on creating a product that was able to last this long before berating them for not indicating on their site (as far as I am willing to spend time searchng) that this member of the MT series (unlike many others, so an inference was not possible) that this model is "maintenance free" which is a cute way of saying it's a Sealed Lead Acid battery with all of the drawbacks of a Flooded and none of the benefits of a Sealed - chief among my grievances the lack of a user-accessible entry point replenishing the electrolyte with distilled water as had been my initial intention to see how much longer the battery's life could be extended beyond its already borrowed time. I would note that according to Interstate's product information page the Group 101 fitment seems to only be used by two models of Cadillac. Looking at their price list and comparing the MT-101 to the notoriously similar and broadly substituted Group 78 model of the same series, one could be forgiven for assuming the 101 configuration was devised solely to impose a 100% snob tax on new Cadillac owners.

This ballache was of course escalated by Cadillac's tendency to play dress-up with their components; one assumes to hide its lowly GM origins but the net result is an unnecessarily tedious process to gain unfettered access to the battery. It should be noted that Cadillacs will tend to sound the anti-theft alarm during low battery voltage events so be sure to have your keyfob out and handy and brace yourself so you don't jump straight into the popped bonnet if and when it trips. Given the Group 101 is a side-posted battery you will, by tugging the battery cable downward and pressing the socket and bolt against the terminal as you disconnect it and then pulling both away from the battery in one deliberate motion once the fastener has completely unscrewed: avoid a lot of solenoid and relay chatter (which doesn't sound terribly healthy), intermittent alarm sounding and most importantly sparking which - I am told - is not a great thing to have happening in the vicinity of a lead-acid battery that could very well be surrounded by hydrogen gas depending on the severity of your battery's misery. Remember to start with the positive cable and then negative.

You can follow this video if a visual reference is helpful:

While the gentleman in the video uses a battery maintainer plugged into the accessory port to retain settings I can speak from experience and also from an owners' forum deep dive which included input from a Cadillac assembly line worker that it is not necessary; some owners report having to re-index the power windows per directions provided on the console but this was not my experience. For fun I rigged a similar device out of a 120V to 12V AC to DC inverter meant for powering portable cooler/refrigerators and an accessory port to accessory port cable salvaged from a device similar to his. Being capable of delivering up to 5 amperes instead of a battery maintainer's typical < ~2 one wold assume it might perform better but I chose to live without it when re-installing what turned out to be a battery impossible to re-hydrate and suffered no ill effects. While the video instructs you to push the ignition button to the "on" position it is not necessary in this particular model as all accessory ports in the cabin are always connected regardless of the state of the ignition switch. Since my dashboard/ashtray accessory port fuse frequently dies (likely from equally frequent use as a good-old-fashioned lighter, like god intended) I plugged into the port hidden in the centre console.

A search through relevant owners' forums will tell you that the preferred, tried, tested and true compatible replacement BCI group size for the 101 is Group 78. Models in this format are, according to the specification, up to an inch taller but, crucially, sport side-mounted terminals (leftmost: positive). This configuration is rather scarce among automotive batteries and I imagine for good reason. I generally find side-mounted terminals annoying, mostly for their tendency to spark while you are installing/removing the cables and in the case of my STS the way they lock the questionably designed battery cover in place, forcing you to remove the cables completely to reposition it and thereby necessitating sparky fun just to get a look at the battery. However, headroom is often an issue and with an additional inch in height (again, according to the specification...) - especially on my STS where the MT-101 had crammed its cover right up against the bottom of the cowl - I was expecting to have to perform some minor surgery or rearrangement just to fit a Group 78 anyway so top-mounted posts were ruled out.

Imagine my surprise when the MotoMaster Group 78 AGM I eventually settled on turned out to have virtually identical dimensions as the MT-101. Remember what I said about the snob tax? ....yeeeeeah. But now I'm slightly ahead of myself.

In spite of the practically unanimous wisdom of the masses I performed the battery replacement due diligence ritual I always do the first time I have to replace a battery in a vehicle that is new to me. After all, unless something goes wrong you only get one chance to upgrade your battery and when you have to cope with the kind of winters I do this isn't the kind of decision one makes with anything short of certainty that you are installing the best possible option available for your money.

The process is straightforward enough:

1. Pull up a list of BCI battery group sizes which contains their dimensions. For this battery I used https://www.jegs.com/Sizecharts/bcigroup.html and google to fill in the blanks on some sizes that weren't included but were referred to during other research I performed.
2. Determine which of your dimensions is the most restricted and which have wiggle room. Note how much wiggle room that is - if you are working with a solid box or sided tray then your absolute maximums have been defined and you must work within them unless you are prepared to make modifications to the vehicle.
3. Compile a list of sizes that are within the limits of your most constrained dimension. For me this was the height of the battery, I allowed about an inch above the current model despite a tight fit under the cowl because I was already working with one of the shortest models.
4. Eliminate those entries on the list that are too big for your more adjustable dimensions. Also drop those that are smaller than your current battery or substantially smaller than your potential maximum - we're looking for an upgrade not a disappointment and volume is the key.
5. Rank the remaining size(s) by order of Cold Cranking Amps (CCA) and eliminate anything with lower CCA than your existing battery. CCA is the most important metric to anyone who experiences winter. CCA will vary for each size by manufacturer, chemistry, and model - even from the same manufacturer. To get a genralized idea of how sizes will tend to perform relative to one another it is helpful to find a manufacturer's listing of all their products which includes the CCA rating of as many sizes as possible. If they do not manufacture a certain size you can fill in the blanks with Google.
6. Make your final decision, which may include considering other metrics like resilience to deep discharge or running capacity.

Starting with the Group 101's metrics, my table looks like this by the time I've run through all the steps:

The sizes marked with a hyphen (-) indicate commonly recommended BCI group sizes substituted for DIN sizes (H*). Unfortunately all the extra work brought me was a little peace of mind; I determined that in my case the best size was in fact Group 78, for reasons not the least of which included four Motomaster variants in stock the next afternoon at Canadian Tires close to me and on the routes of already planned errands.

Though I tend to buy Kirkland Signature batteries for Costco's amazing return policy I didn't notice any Group 78 AGM batteries on the photos I took at my local the other day (a necessary strategy if you are going to shop your local costco's battery inventory from home) and Canadian Tire's warranty and return policies on batteries are also excellent.

Among the most important and simplest upgrades you can make to a vehicle that operates in parts of the world that experiences the phenomenon known as "weather" is to switch from a typical Flooded Vented Lead Acid (VLA or commonly just "flooded") battery to an Absorbent Glass Matt (AGM) battery. AGMs, together with Gel Cells (commonly called "Sealed Lead Acid" or SLA) are one of the two types of Valve-Regulated Lead Acid (VLRA) format. These batteries - except under extreme circumstances of failure - do not leak, can be mounted sideways, are resilient to vibration, crank engines in the coldest temperatures, will rebound from deep discharge far better than any other lead plate format - even those that are not explicitly advertised as intended for "deep cycle" - and as with maintenance free batteries you can not (will not need to/will compromise the package if you) add additional distilled water in the future. While extremely old AGMs would benefit from that - as they do offgas hydrogen and decompose very slowly it is an entirely different context as conventional flooded batteries.

While AGM batteries cost tangibly more (from +CAD$60 and up) they are a sensible and worthwhile investment. It is not necessary to buy name brand AGMs like those manufactured by Maxima; the extreme premium they charge erases any advantage they might - and probably don't - have over more "generic" manufacturers like those that whitelabel for Kirkland Signature and Motomaster.

Even if you don't operate your vehicle in rugged, masculine climates - as your humble author does - it is still very often worth considering replacing your OEM flooded lead acid battery with an AGM when its time inevitably comes. Aftermarket electronics like an upgraded head unit, UHF radio or feeble compensatory sound-system can tax your battery especially if your alternator is older or declining. An AGM battery is particularly advisable if you find your sound system requires a capacitor inline with the power connection. Even without these additions, the longer service lifetime and higher performance can likely offset the steeper initial investment. Also, I've heard that at least once in one's lifetime it is common to accidentally leave the lights or some other component on or forget to unplug a device that has been connected to an always-on accessory port. That's never happened to me, of course. Despite what one may have heard. Yup.