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Big Dog Motorcycles closed its doors in 2011, which changes how you shop for parts but not what the bike needs. Across the Bulldog, Chopper, Mastiff, Pitbull, K-9 and Ridgeback models — including the 1442 cc and 1750 cc bikes — the factory battery is a YTX20L-BS, a 12 V, roughly 18 Ah AGM in a 175 × 87 × 155 mm case with the positive terminal on the left. That single part number is what makes this easy: it is one of the most widely produced motorcycle batteries in the world, so a discontinued marque is no obstacle at all. Three things separate the options. Cranking amps — an S&S or Big Dog-spec V-twin with high compression wants real cranking current, and this is where the cheap batteries reveal themselves on a cold morning. AGM versus lithium — AGM is heavy, cheap, forgiving and works with any charging system; lithium saves roughly 10 lb, lasts longer, and needs a lithium-compatible charger and a healthy regulator, or it will disappoint expensively. Terminal orientation — the L in YTX20L-BS denotes the terminal layout, and a YTX20-BS without the L has the terminals reversed, which puts the cables under strain or simply out of reach. Ten batteries below, from the factory-equivalent to the lightweight upgrade.
| # | Product | Price | Where to buy |
|---|---|---|---|
| 1 | Yuasa YTX20L-BS (YUAM320BS) | $110–$145 | Amazon · eBay |
| 2 | Yuasa YTX20HL-BS High Performance (YUAM320LB) | $130–$170 | Amazon · eBay |
| 3 | MotoBatt MBTX20UHD | $95–$135 | Amazon · eBay |
| 4 | Deka / East Penn ETX20L | $90–$125 | Amazon · eBay |
| 5 | Interstate CYTX20L-BS | $95–$135 | Amazon · eBay |
| 6 | Power Sonic PTX20L-BS | $70–$100 | Amazon · eBay |
| 7 | Mighty Max ML20L-BS | $50–$75 | Amazon · eBay |
| 8 | Banshee 20L-BS AGM | $55–$80 | Amazon · eBay |
| 9 | Antigravity ATX20-HD Lithium | $290–$390 | Amazon · eBay |
| 10 | Shorai LFX Lithium (YTX20L-BS equivalent) | $250–$340 | Amazon |
The picks, in detail
Yuasa YTX20L-BS (YUAM320BS)
The original-equipment answer. Yuasa is what most of these bikes shipped with, and it remains the benchmark for AGM reliability.
- 12 V, 18 Ah AGM construction
- Factory activated or dry-charge versions
- Sealed and maintenance free
- Correct L terminal orientation
- The reference standard — everything else is compared to it
- Consistent quality across production runs
- Widely stocked at dealers and online
- Priced well above the budget AGMs
- Heavy at roughly 15 lb
Yuasa YTX20HL-BS High Performance (YUAM320LB)
The same case with more cranking amps — the sensible upgrade for a high-compression V-twin that turns over slowly when cold.
- Higher cold cranking amps than the standard YTX20L-BS
- Identical case dimensions and terminal layout
- Sealed AGM
- Factory activated options
- Direct drop-in with meaningfully more starting current
- Best choice for a big-inch motor with high compression
- Same Yuasa quality and support
- Costs more than the standard version
- The extra capacity is wasted on a stock-compression bike
MotoBatt MBTX20UHD
The clever one — a quad-terminal design that lets you run accessories without stacking ring terminals on one post.
- Four terminals for easier accessory wiring
- Higher capacity than the standard YTX20L-BS
- Factory sealed AGM
- Two-year warranty
- Quad terminals genuinely simplify a bike with added lighting or audio
- Higher capacity than the part it replaces
- Good warranty for the price
- Slightly different terminal geometry needs a check against your cable routing
- Less brand recognition than Yuasa
Deka / East Penn ETX20L
American-made AGM from a manufacturer that supplies far bigger names — often the best quality-per-dollar here.
- AGM construction, sealed
- Made in the USA by East Penn
- Direct YTX20L-BS equivalent
- Strong cranking performance
- Build quality close to Yuasa for noticeably less
- Domestic manufacture with good warranty support
- Consistent performance in cold weather
- Distribution is patchier than the big brands
- Packaging and labelling vary between sellers
Interstate CYTX20L-BS
The one you can buy today in person. Interstate distribution means a dead battery on a Saturday is a drive rather than a wait.
- Direct YTX20L-BS equivalent
- Sealed AGM
- Nationwide dealer network
- Warranty handled locally
- Available same-day almost anywhere in the US
- Local warranty handling rather than a shipping claim
- Reliable mid-tier performance
- Priced above equivalent online-only brands
- Performance is solid rather than outstanding
Power Sonic PTX20L-BS
A well-regarded mid-tier AGM from a manufacturer with a long standby-power pedigree.
- Sealed AGM, maintenance free
- Direct YTX20L-BS equivalent
- Ships factory activated
- Good cold cranking rating
- Better than budget performance at a near-budget price
- Arrives ready to install
- Solid warranty terms
- Less common in physical shops
- Not as long-lived as Yuasa in hot climates
Mighty Max ML20L-BS
The budget AGM. It will start the bike and it will not last as long — a reasonable trade if the bike is stored more than it is ridden.
- Sealed AGM construction
- Direct YTX20L-BS replacement
- Ships activated
- One-year warranty
- Cheapest workable option here by a wide margin
- Fine for a bike that sees light seasonal use
- Widely available online
- Noticeably shorter service life than the premium AGMs
- Cold cranking falls off faster with age
Banshee 20L-BS AGM
Budget AGM with an unusually long warranty — the value pick if you are prepared to deal with an online claim.
- Sealed AGM
- Direct YTX20L-BS fit
- Four-year warranty on some versions
- Ships activated
- Warranty term well beyond what the price suggests
- Adequate cranking for a stock motor
- Cheap enough to keep a spare
- Quality varies between production batches
- Warranty is handled online only
Antigravity ATX20-HD Lithium
The weight-saving upgrade — roughly 10 lb lighter than the AGM it replaces, with far better shelf life between rides.
- Lithium iron phosphate chemistry
- Roughly a third the weight of an equivalent AGM
- Very low self-discharge
- High cranking output for its size
- Weight saving is immediately noticeable on a chopper
- Holds charge for months in storage
- Longer service life than any AGM here
- Needs a lithium-compatible charger — an ordinary trickle charger will damage it
- Cold-weather starting requires a warm-up cycle before cranking
Shorai LFX Lithium (YTX20L-BS equivalent)
The other established lithium option — light, long-lived, and supplied with foam shims to pack out the larger factory tray.
- Lithium iron phosphate chemistry
- Foam shims supplied for tray fit
- Very low self-discharge
- Substantial weight saving
- Foam shim kit solves the loose-in-the-tray problem properly
- Excellent storage behaviour over winter
- Strong cranking for the weight
- Requires a lithium-specific charger and a healthy charging system
- Expensive relative to the AGM it replaces
Buying tips
- Check the L. A YTX20L-BS and a YTX20-BS are not interchangeable — the L denotes the terminal layout, and getting it wrong means the cables reach the wrong posts. On a Big Dog the positive should sit on the left as you face the battery.
- Before you blame the battery, measure the charging system. Big Dog bikes are old enough now that a failing regulator or stator is a common cause of what looks like a dead battery, and a new battery installed into a bad charging system dies in weeks. You want roughly 13.8–14.5 V at the terminals at cruising rpm.
- Lithium needs a lithium charger. This is not a preference. A standard lead-acid trickle charger applies a desulphation pulse and a float voltage that lithium chemistry does not tolerate, and it will shorten or end the battery life. Budget for the charger alongside the battery.
- Lithium also starts poorly when genuinely cold. The usual technique is to turn the headlight on for thirty seconds before cranking, which warms the cells enough to deliver full current. If you ride in real winter, AGM remains the easier choice.
- Big Dog closed in 2011, so buy on the part number rather than the model name. Any seller listing a YTX20L-BS with the correct dimensions and terminal orientation is selling you the right battery, regardless of whether Big Dog appears anywhere on the listing.
As an Amazon Associate and eBay Partner, toptenpick.com earns from qualifying purchases. Prices and availability are subject to change.