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How to Maintain Deep Cycle Batteries

The Battery Genius knows a thing or two about maintaining your deep cycle batteries.  I’m going to teach you now, but before I do, I want you to realise the importance of doing ALL of these things in order to make your batteries last their longest.  Don’t be that guy in Duluth or Cleveland that does ONE or TWO of these things and then whines because his batteries don’t last. Flooded Deep Cycle Batteries require love.  So I will tell you how to love your batteries well and How to Maintain Your Deep Cycle Batteries.

First, I want all of you to raise your right hand and repeat after me:

“I, (state your name), solemnly swear to trust The Battery Genius do what he says so that my batteries will last a really long time. I will not flake and only do a couple of these things, only to regret it later.”

First things first; New batteries should be given a full charge before use. Yeah, yeah, I know —  your new batteries come from the manufacturer ready to rock-n-roll, but ALWAYS charge your batteries fully before use, please.

New batteries need to be cycled several times (AND NO GREATER THAN 50% Depth of Discharge) before reaching full capacity and hitting their sweet spot — Usually about 20-50 cycles should do it.  Most importantly, be sure to limit how deep you discharge these babies when you first start using them. That’s right.  I said Babies.  Treat them like babies. You have to nurture and work them into the rhythm of how you will use them before you can really start working them.

Batteries should be watered AFTER charging UNLESS plates are exposed before charging.  If exposed, plates should be covered by approximately 1/8″ of water.  Check water level after charging.  The water level should be kept 1/4″ below the bottom of the fill tube in the cell cover.  A Pro-Fill watering system can eliminate the guesswork of proper fill levels and really help with maintaining  your batteries.

Water used to replenish batteries should be distilled or treated to not exceed 200TDS (Total Dissolved Solids…Parts Per Million).  Particular care should be taken to avoid metallic solids like iron.  You’ll hear a lot of old salty battery dudes sitting around your local garage or battery shop sayin’, “If it’s clean enough to drink, you can put it in yer battery.”  Don’t listen to him.  Even if he makes fun of you for driving a mini-van and knowing nothing about batteries, DON’T put tap water or spring water in your battery.  Use distilled water.  You’ll thank me.

Batteries should NEVER be discharged below 80% of capacity (approximately 1.8 volts per cell under normal load;  1.98 volts open circuit; 1.145 specific gravity).  ”But, Battery Genius, aren’t Deep Cycle Batteries designed to be discharged and recharged over and over,”  You ask?  Short answer, Yes.  However, anything past 80% DOD is only working them harder than they were designed and you risk speeding up sulfation of the battery plates. Knowing and using the proper battery for the job will help avoid excessive discharge.

Here’s a hint on how to know whether or not you have the right battery bank for the job — If your batteries won’t completely charge in 8 hours, you’re not using enough battery.

In situations where multiple batteries are connected in series, parallel or series/parallel, a replacement battery(s) should be of the same size, age, and usage level as the companion batteries.  DO NOT REPLACE JUST ONE BAD BATTERY!  Never put a new battery in service with a pack which has 50 or more cycles.  Either replace will all new batteries, or use a good used battery(s) with same cycle use.  If you use an older battery with the new, the older battery won’t be able to keep up and will ultimately die or bring the new batteries down to its level, shortening their life.

Please, please, please, test your batteries every now and then (Four times a year).  Periodic battery testing is an important preventative maintenance procedure.  Hydrometer readings of each cell (fully charged) gives an indication of balance and true charge level.  Imbalance could mean that need for equalizing and is often a sign of improper charging or a bad cell.

Voltage checks (open circuit, charged and discharged) can locate a bad or weak battery.  When checking specific gravity, it is possible to convert the reading to what the open circuit voltage should be.  The formulas is to add .84 to the hydrometer reading and multiply by the number of cells.  Thus, a fully charged battery with a reading of 1.265 SpGr will have an open circuit reading of 12.63 volts.

Load testing will pick out a weak or bad cell when other test methods fail.  The point is to look the abnormal.  A weak cell or battery will cause premature failure or erratic function of companion elements such as batteries, navigation systems and other electronics.

DO NOT use a mismatched charging source of any type.  An undersized charger will never get the job done, no matter how long it tries.  An over-sized charger may cause excess gassing and heat that could possibly result in thermal runaway, and explosion! Explosions are bad.

Batteries are like muscles, Inactivity can be harmful to Deep Cycle Batteries.  If they sit for several months, a “boost” charge should be given; more frequently in warm climate (about once a month) than in cold climates (every 2-3 months).

Lastly, just like people, as batteries age their maintenancerequirements change.  Generally, their specific gravity is higher.  Gassing voltage goes up.  This means longer charging times and/or higher finish rates (higher amperage at the end of charge).  Usually, older batteries need to be watered more often or their capacity may decrease.

Hopefully, you’re a little wiser today about your Deep Cycle Batteries than you were before you found The Battery Genius. To BUY Deep Cycle Batteries or any of the maintenance items mentioned here, visit Powerstride Battery for all of your battery and battery related accessories.


  1. I have had some issues with w combo marine battery (deep cycle starting). I have over time seen this battery go to 1.5V. After a typical charge using my 2 A trickle charger it only reaches 12 V. I am using this battery to power eletronics and downriggers on my boat. There is a slow load of about .1A that slowly discharges the battery – then all at once it drops to 1.5 V.

    The last time this occurred I removed all load, placed the battery on charger and within less than a minute it showed 11+V. I disconnected it from charger and left sit for approx 24 hrs. Then I took it to the place I bought it – it tested 9.5V and needed a charge before test. After charge it showed 12.8V and load tested good……I managed to get a new replacement because I told them I had no confidence in this battery.

    So – do you think I have a dead short somewhere or once a battery reached a certain discharge point the bottom will drop out….and then still be able to be fully charged again?

  2. RedRob /

    Hi! I had a “deep cycle marine battery” sitting in my basement unused for the past four years. I charged it and used it camping two weekends ago to pump my air mattress (with inverter).. it worked! But by day 6, it couldn’t give enough juice to the inverter to pump the bed. (initial full-fill + about 12 partial-fills since we added air in the middle of the night).

    Does this sound like a reasonable charge the battery is providing, or is it showing its age? (I didn’t check water levels in the battery)

    Any advice?

  3. Jason /

    I’ve recently bought a golf cart that was not charged up at all when I bought it, but I’ve added water and reconditioned the batteries to take a charge. 6 volt batteries showing between 6.16 & 6.25 volts. The charger is only putting out about 2 volts currently and has been plugged in all night and it seems to be gasing a lot. Should I just leave it plugged and let it try to get a full charge or do you think the batteries are done? 4 year old batteries.

  4. The Battery Genius /

    Batteries are probably done, sadly. Sorry :(

  5. The Battery Genius /

    @RedRob — Sounds sulfated and showing its age.
    Time for a new Battery. Check the electrolyte first, but it sounds like toast to me.

  6. The Battery Genius /

    @Elijah – Sounds like you may need a three bank. But I’d look into a charge isolator switch… something seems amiss here.
    What is the Amperage charging the two batteries? they may need longer to charge.

  7. My trolling motor for my boat states to run it off of 24volts. Once I run the batteries in a series for the trolling motor, can I still power other electronics(radio, gps, etc.) connected to one battery without effecting them or frying them?

  8. The Battery Genius /

    @Jim – It is entirely possible you have a short in the battery. More likely, however, is that the battery is sulfated and the 2Amp charger is not enough to to charge it due to the internal resistance to the battery. The larger the battery, the larger the charger needed. The OLDER or LOWER the voltage the battery, the larger the charge is needed.
    Think of a battery charger like a air pump to a flat tire. The first few pounds of pressure go in easily, but the more pressure builds up, the more powerful a compressor needs to be to put air into the tire.
    We see this routinely with older, more severely discharged, or UNDERCHARGED batteries.

  9. The Battery Genius /

    @Erik – typically you can. But it is not recommended. Get one set for your trolling motor and another auxiliary battery for other electronics.

  10. Mike Zimmer /

    I have a three bank charger for my batteries and the deep cycle battery gets hot when on charge . The battery is seven years old is it time for a new one, it is in a series with another battery for my 24 volt trolling motor. Waiting for an answer..

  11. The Battery Genius /

    @Mike Zimmer – All batteries get hot during charge. However, if your battery is exceptionally hot, it could be due to sulfation causing extreme internal resistance. Sounds to me like it’s time for a new batteries. You don’t want to switch just one.

  12. My golf cart won’t start (2007 CC precedent) after putting water in batteries w/watering system. I decided to charge it. Any advice?

  13. I have 16 6V T-105s in series/parallel. They are charged by solar, turbine, and back-up generator. All summer they held charge well and met our power needs… Our generator would only come on once every few weeks. Over the last month, however, the generator has been kicking on every day. Even after a full day of sun and charge, if we start drawing significant amperage (well pump, TV, washer, etc) the generator instantly kicks on. Does this indicate aging batteries?

  14. The Battery Genius /

    @Jess – It certainly CAN indicate aging batteries, provided they have been properly maintained (electrolyte, charging at the proper rates and not severely discharged.) Or, it could indicate the need for Equalization.
    Give us a call – TOLL FREE – at (877)576-9379 or call the Powerstride closest to you and we’ll send you some charging profiles to help!

  15. The Battery Genius /

    @Jeri – Sounds about right. A full charge can’t hurt!

  16. The Battery Genius /

    @Jess — Sorry for the delay. It can indicate age. It can also mean that with temperature variations, you may need to charge at a higher rate… Higher rates for colder days; lower rates for warm days. During fall and winter months, your batteries will discharge faster and recharge slower at lower rates.
    Hope that helps! Sorry again for taking so long to reply!


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