Battery pack

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RoboMower battery packs consist of a plastic shell holding two 12-volt deep cycle lead-acid batteries wired in series. This provides 24 volts to power the mower. Each battery pack has a set of large metal strips on the side that make contact with the mower when the battery is in place. A protection fuse is installed on the same side of the pack. Some battery packs also have a small connector that can be used by an external charger.

Battery Charging

Batteries are charged while installed in the mower using the supplied power pack or dock. The typical charging time is 20 to 26 hours. An optional, external quick charger is available that will charge the battery pack independent of the mower in four to six hours. You may have heard that "fast charging is not good for a battery's life". This is true, but the external "fast chargers" we use are generally 3A which is actually in the "normal charge" range. For a 20AH battery, the charge rate would have to be around 6A before even beginning to enter the range which may reduce battery life.

High operating and storage temperatures significantly reduce battery life. Avoid charging batteries while they are warm, especially on the fast charger. Many people with a large yard buy two batteries and immediately charge the first while swapping in the second, and then immediately swap the first one back on the mower when charged. The temperature buildup from this is excessive and reduces battery life. Follow the Robomower recommendation to put the auto-charging docking station in a shady area.

NEVER PLACE A BATTERY ABOVE 120F ON A FASTER CHARGER. This may result in a thermal runaway condition which will destroy the battery. This has been reported as literally melting a battery which was hot off the mower on a hot day and placed directly onto the fast charger.

Opening a Battery Pack

Take your palm and press on the center of the lid and pull up and out slightly on the side of the lid near the tabs visible on the side.

Alternatively you can simply lift a bit on the front of the lid and open the door but access to the wires isn't quite as easy.

RL500 battery pack with the front open to show the batteries inside and the plastic clips that hold the case closed. (The original batteries have been replaced with a different model.)

Battery Pack Wiring

Here is a front and top view of a RoboMower battery with the top removed to show the wiring. When replacing batteries, be aware than a few batteries are manufactured with the positive (red) on the left instead of the right. Always put the red cable on the red terminal! Do not unplug the end of the wire attached to the case parts, they have no colors on the case and you may mix them up.

Battery pack internals

Making your own External Charger

Since the Electric Bike craze, 24v SLA "smart" chargers are a dime-a-dozen. Almost literally. Common sizes are 1.5 amp to 4 amps. Robomower's official "fast" charger was 3 amps. 24V bike chargers on eBay. Starting at ~$13. Avoid getting a LiFePO4 or NiMH charger, it that's not in the title though, it's an SLA.

There's nothing wrong with using a 1.5 amp charger, it's slower and could take 14 hours to recharge a 20AH batt. If you are not planning to use your batts a second time that day, this is fine, better even. It will charge them more gently overnight and they may last a bit longer.

You will need to splice a connector to connect to the Robomower pack. The "Charging Connector" on the Robomower pack is a 3-pin .093" Molex:

[1] Fry's has them here. These have "ears" for panel mounting, which you won't be needing, except the ears may make it easier to grip to pull it off.

[2] These are the ones at Mouser. You will need to buy sockets (pins), which are sold separately.

[3] One without "ears".

[4] Another one without "ears", not sure what the difference is.

[5] Here's some sockets.

[6] More sockets.

Don't get sockets with gold plating, the mower packs use tin-plate, and you can't connect gold to tin. There are several different sockets in the .093 size with tin plating, the difference is all in the wire diameter they're designed for AFAIK.

Alternately, you could wire in an XLR jack, the type used by these chargers, into the pack itself and make it come out the handle cutout.

Tracking Charged and Uncharged Batteries

User:Dougs likes to keep a bunch of extra batteries charged and ready to go by using an external charger. He suggests using paper binder clips as markers to indicate which batteries are charged.

Batteries charged with the external charger. Paper binder clips are used to indicate which batteries are charged.

Battery Maintenance

All sealed lead acid batteries are subject to reductions in performance and total lifespan if stored at less than complete states of charge, and degradation is far more rapid at significantly lower charge states. Even when not used but maintained ideally a battery still has a finite lifespan, perhaps 5 years, thus it is a bad idea to buy extra batteries to get a bulk deal in anticipation of needing them much later. Battery life is far greater when used and in particular stored in cooler temperatures. If a docking station is used it would be best to place it in the coolest location possible. If practical, moving batteries inside if the outside temperature is excessive may extend their life. Batteries, even the deep cycle type, yield more cycles when used at shallower cycles. As a non-deep cycle example, the Tysonic specification says 1500 cycles at 30% Depth of Discharge (DoD), 600 at 50% DoD, and 200 cycles at 100% DoD. Considering that a 50% DoD is only providing 50% of the runtime per cycle, we can calculate total runtime equivalency as 450, 300, and 200 lifetime discharges when used regularly to 30%, 50%, and 100%. As a deep cycle example, the EVP20-12 specification says 1500 cycles at 30% Depth of Discharge (DoD), 600 at 50% DoD, and 300 cycles at 100% DoD. This equates to 450, 300, and 300 full cycles. As yet another factor, the battery provides a higher voltage during shallow discharges and thus the mower is more effective during this usage. The Robomower does not have a programmable limit for voltage/depth of discharge. The user may program the runtime to limit depth of discharge.

Winter Battery Storage

Extended storage and depleted charge state will shorten the life of the batteries. The batteries should be fully charged then stored outside of the mower in a dry area at room temperature or cooler, such as a basement. Batteries should never be allowed to freeze. A fully charged battery will not freeze until -40C, but a partially discharged battery will freeze in less severe conditions, a totally discharged battery will freeze at only 0C. It is recommended that the batteries not remain on the charger all winter but to charge them once or twice during that time since batteries gradually self-discharge over an extended period of time. They should be fully charged again before use in the spring.

Replacement Batteries

The battery pack contains two sealed lead-acid batteries. Once the battery pack has been opened, it is fairly simple to replace the batteries. The two 12V batteries should be the same size in order to fit within the battery pack (181mm x 168mm x 76mm, 7.13" x 2.99" x 6.57"), known as a "half U1" within the industry. Most of the half U1 batteries come equipped with a 5mm bolt-hole terminal know as B1 and you will need to reuse the adapters bolted onto your pack's old B1-style terminals which adapt it to the quick-connect cables inside the pack. There are half U1 batteries which use a terminal called "T2" which the pack's quick-connect cables can be plugged straight onto, however this style is rarely found on quality high-rate deep cycle batteries and offers no technical advantages.

There is a significant amount of variation in battery make and model within the half U1 case. The Robomower RL800 draws approximately 3-5 amps while mowing, and as high as 7-9 amps when spinning its wheels against an obstacle. While this is not an especially high rate that can damage batteries, many batteries not designed for "high rate" use are only capable of delivering significantly fewer amp-hours than their stated capacity even at this current draw. For example, the BB Battery "General Purpose" BP17-12 is rated at 17AH when drained slowly over 20 hours, but at a 5 amp rate can only deliver about 12.5AH. Get a battery designed for "high rate"/"high current".

The "deep cycling" or "high cycling" batteries which are specified to last for at least 300 cycles at 100% Depth of Discharge (DoD) would be the best choice. Many others are designed primarily for standby use in UPSs and such and are designed to last perhaps half that, although their high current delivery capabilities in the Robomower degrade even faster yielding very little effective service life.

All specifications are potentially unreliable, sometimes batteries are described as "specifications subject to change without notice". Unfortunately the use of Chinese manufacturers in the environment of high lead prices and the difficulty of analyzing battery performance regularly leads to a great deal of consistency questions. At the same time stories of high performance without actual controller, long term measurements or isolated stories of premature failure are also difficult to draw conclusions from. Also very significant, battery capacity increases by almost 10% in the first 100 full cycles or so, some mfgs rate by the first-cycle capacity and rate conservatively, others rate by the peak capacity in mid-life and exaggerate on top of that. However, I can only go with what the spec sheet says and that's the basis for the evaluations for each battery represented below.

Friendly Robotics changed the low voltage shutoff limit in later models to prolong the batteries' total service life, but this reduced the runtime possible on a single charge for the same battery. Batteries will provide less capacity at lower temps, 0C is approx 15% less capacity than 25C, and battery capacity actually increases by about 10% during the first 100 or so complete cycles of use. These factors make it difficult to compare different users' experiences with particular batteries.

A thread dedicated to finding deals on replacement batteries is available at

EV Batteries

These batteries are specifically designed for EV service- hard, deep cycling under field conditions and should be regarded as the gold standard of Robomower batteries. These typically perform better than batteries designed as "high rate" in the long run, even though they may have similar lifetime ratings.

Tempest TD22-12

Rated for Deep Cycle, and has the highest capacity ever claimed, yielding 20AH at the 5A rate. BatterySpec does 20H-rate load testing on the pallets they get from Tempest and they say it does put that out. It weighs a full 15lb/batt and that's a good sign (lighter batteries suggest we've been short-sheeted on plate material). It is also the CHEAPEST EV battery on the market right now. However, as of yet no one from the Robomower group has tried it and as such it is not "proven". I'm still giving it the "overall best buy" rating by far, both in terms of performance AND price, unless reports come in to say otherwise.

UPDATE: among the Robomower email list folks, a number of people have used them. Results are mixed. Initial mowing times seem good but not outstanding, and early capacity decline and early failures have been seen. The same seems true of any battery, but taking into account the modest number of people using them, they're not looking like a "great" high performing/high reliability batt. Still, good for the price.

  • Dealer: BatterySpec (note: BatterySpec's other 18AH "Robomower" pair is a GP battery. That will actually not perform well as a mower pack)

BB Battery EB20-12/EB24-12

This new BB product line is labeled as "Mobility" batteries. Rated at 300x 100% DoD cycles and specifications indicate approx 18.75AH of usable capacity (extremely high) at 5A. Both the EVP20-12 and EB20-12 are rated for 300x 100% DoD cycles, but I did speak with a BB Battery salesperson who said they're slightly thicker plates than the EVP, designed specifically for electric bikes ("EB"), and in the long term would probably perform better than the EVP in this application. These are cheaper than EVP20-12 with some online vendors, but Digikey seems to have the same price for either one. EB20-12 has been used by a number of Robomower group members, who have generally reported excellent performance. Has not been used long enough to prove its ultimate lifespan.

UPDATE: BB Battery now has an EB24-12 in their product line, with 20% better specs all the way around, in the same size case! However, they are not available yet. ETA "Summer of 2010".

CSB Battery EVX12200

This is what CSB specifically recommended for Robomower service. 20AH base capacity, 350x cycles at 100%, highest ever claimed. Specifically designed for personal mobility. Delivers 14.5AH at the 5A rate, which is on the low side. No one from the Robomower group has used these, performance is unproven. Altex Electronics (Texas) carries these on-the-shelf at reasonable prices.

Enersys (Yuasa) NPX-80

Enersys bought the battery business of Yuasa in 2000. The NPX-80 is their current deep-cycle offering. I am unclear on whether this belongs here or in the "High Rate" batteries, but people do recommend them. They are rated as 250 100% DoD cycles. They do not state a voltage versus current rating, however, their watts-per-cell at the 1hr rate (far higher than the Robomower uses) delivers 25 WH/cell, very close to the EVP20-12 which delivers 27 WH.

High Rate Batteries

These offer high initial usable capacity, but I've had techs from two different companies say that under field conditions they simply won't perform as well in the long run as the EV batteries, even though the deep cycle life span ratings are not that bad. The extremely high rates, their main selling point, will not be used by the mower, although this makes almost all of the "nominal" capacity available at the 5A rate. Their high capacity degrades quicker under deep cycle use, especially in harsh conditions.

BB Battery EVP20-12

These seem to be touted by many as one of the better deep-cycle batteries available, rated at 300 100% DoD cycles and specifications indicate approx 18.75AH of usable capacity at 5A. They have been mentioned on the Yahoo Robomower Mailing List. They have also been mentioned as a good lead-acid battery to use for converting the Toyota Prius to be plug-chargable. Specifications: EVP20-12 Specifications

CSB Battery HR1290W

Highest capacity of all (23AH nominal). Delivers whopping 21AH at the 5A rate, highest ever claimed. 260x cycles at 100% DoD. Might require slight modification of Robomower battery cables due to recessed terminal type.

CSB Battery HRL1290W

23AH nominal. What data does show is it delivers 18.45A H at 12.3A, so at 5A we should be above 20AH, in fact this appears it could be better than the HR1290W. 260x cycles at 100% DoD. Might require slight modification of Robomower battery cables due to recessed terminal type.

CSB Battery HRL1280W

20AH base capacity, 260x cycles at 100% DoD. Delivers 18AH at the 5A rate. Might require slight modification of Robomower battery cables due to recessed terminal type. Bargain price.

General Purpose stuff, not as highly recommended

MK Battery ES17-12

This is the battery Friendly Robotics often supplies. It is a very poor performer at Robomower currents, yielding approximately 12.7AH at 5A. It is rated for 250x cycles. Frankly, the lack of an extremely detailed spec sheet or website, and the stuck-on paper labeling of their product (which in my case contained no indication of capacity) doesn't give me a lot of confidence. For that matter, their own spec sheet contradicts itself, claiming 18AH as the 20-hr capacity but the discharge graph says 17AH, which is consistent with the product name.

BB Battery BP20-12

BB's general purpose line. It is only slightly less capacity at the 5A rate than the EVP20-12/EB20-12. However, it is a general purpose product line, not specifically described as a deep cycle like the other two and has no deep cycle life cycle ratings in its specs. Since Digikey sells EVP20-12/EB20-12 at the same price there is no reason to use this product for the Robomower application.

Universal Power Group UB12220

This is a high capacity (22AH nominal), moderately high current battery which is selling fairly cheap online. Its specifications show about 17-17.5AH at 5A discharge rate. A UPG representative has described them as deep cycle, however, the official specifications do not describe its cycle life.

Power Sonic PSH-12180/PSH12180FR

21AH, 17.1AH at 5A rate. No life cycle data. At 13.2lbs, this is a bit "light" for 21AH so this could have thin plates not well designed for repeated deep cycling.

Fullriver DC17-12/DC20-12

An AGM deep cycle being provided by Robomower at least in Belgium. It has a stated ~300x 100% DoD rating which is excellent. However, its high current delivery is poor: 12AH at 5A rate. The 20AH provides no substantial gain in runtime, again providing about 12AH at 5A, which is unusual.

Tysonic TY-12-18

I (User:garsh) have tried these, and they worked for less than a year before they would no longer run the mower for a full 2-hour run. But they seemed somewhat promising, and it would be nice to hear if others have had a better experience with these batteries.

User:Danny Miller These have a low 5A current capability and only 200 100% DoD lifespan. This is simply another "general purpose" batt, I would not recommend these at all for this application.

Specifications: Tysonic TY-12-18 Specifications


Yuasa NPC17-12

These were the batteries that originally came with my (User:garsh) mower. They are mainly sold in Great Britain. I have not found anyone who sells these batteries in the U.S. This is a shame, as it appears to have the best cycle service life, even better than the BB batteries. User:Danny Miller They have a higher cycle rating of approx 350 100% DoD cycles. However, at 5A it it rated for only about 12.5AH of usable capacity yielding a lower total service life than BB's EVP/EB series on paper.

Specifications: NPC17-12 Specifications

Werker (Batteries Plus) WKDC12-20NB

User:SiliconSlick found these recently at the local Batteries Plus. WK=Werker (store brand). DC=deep cycle. 12=12V. 20=20AH. NB=nut/bolt connector. I decided to buy locally since my last experience with a discount web site (Battery Universe) ended up not being worth it (see thread above and caveat below) when one considers a) the inferior item actually shipped, and b) the cost of return shipping. So far, my RL500 has run for 5 hours nonstop on a fresh charge, far exceeding any batteries I previously had (original MK ES17-12s and Yuasa NPC17-12s and the Rhino SLA17-12s that Battery Universe sent me instead of B&B EB20-12s). Can't find any specs but at right around $75-$80 each (I paid $70 since I was buying lots of different batteries) I'm hoping they last a while. At least I'm sure I got what I paid for (FWIW). The salespeople there said it was a relatively new (September 2010) addition to their inventory so not all locations might have them yet. (I'm still amazed there are stores now that only sell batteries... and had every battery I needed/wanted to buy.) [Note: this might be more properly placed in the EV or High Rate categories above but given a lack of specs/data I put it here.]

Buying Batteries

In the last two years, lead prices have skyrocketed and raised battery prices. Old archived discussions of price are outdated. Take the shipping cost to your zipcode into consideration. There are possible problems of "old stock", some batteries may have notable degradation after sitting on a shelf uncharged for more than 6 months. Many people desire to avoid shipping by dealing with a local battery dealer, but the prices are typically much higher even when considering shipping savings and it is unlikely to find an appropriate high rate/deep cycle battery at all without special-ordering.

Some discount battery vendors, even those with well designed websites, have done such things as list a product as "Replacement EVP20-12" when they actually substitute a completely different general-purpose battery in a half U1 case with neither the deep cycle nor high rate capabilities of the EVP20-12. They may try to explain this is an "equivalent". Before spending money on batts online, ALWAYS contact the vendor by phone or email to get an assurance of the actual brand and exact model # of the batt you will receive, and ask for their "date code" (industry term) to determine if they are fresh or not, within 6 months is great. Digikey and Mouser are large, reputable high volume electronics industry vendors which would not keep old stock around and would never substitute which typically ship for a very low price. Their price with shipping has consistently undercut even "discount" battery vendors.