AGM battery charge confusion

[nicknight]

I have 2 small AGM batteries I use for onboard electronics, my draw is up to 750mA @12V. The draw is constant but I am maybe doing 5 hours a day for 5 days.

So I need to charge them.
I am looking at a small 7.5W Solar panel that will output at 12V, yes it is small charge but it is on sale and I need this for tomorrow and I think this means it doesn't need a regulator.

I read I can discharge these to 60%. Some guy at the store told me I should only discharge them to 70% and charge them as frequently as possible.

Is 60% to low?
Can/should I just leave the small solar panel on and connected directly all the time, even when running electronics?
What am I going to do for this 5 day trip?

 

[ondarvr]

That small panel isn't really going to make a difference, it doesn't put out enough power to do much more than keep an already fully charged battery maintained. Charging up two significantly discharged batteries takes a good sized solar panel and a lot of sun.

 

[spoilsofwar]

Why not wire them into the boat's charging system? Or does one not exist?

As ondarvr mentions, a small solar panel like that is, under best conditions, a maintainer - not a charger.

Your best option, assuming you can't charge via the boat's main engine, is to have an onboard charger and find somewhere to plug into to charge the batteries.

Without knowing the exact model of your batteries, it's impossible to say what level of discharge is acceptable. If they're quality deep cycle AGMs, they should tolerate repeated 80% discharges. Obviously, you will get longer life out of them if you keep it to 50% or better and keep them charged, but I digress.

 

[nicknight]

I never heard of a deep cycle AGM batter that would allow 80% repeated discharges.
This is for a very small watercraft. I don't expect a small unit to charge them from a significant discharge.

Can/should I just leave the small solar panel on and connected directly all the time, even when running electronics?
or maybe I can just let it discharge to 60% by day 3 and let it sit for a few days? letting an AGM sit discharged at even 10%-40% for a few days is bad, right?

 

[spoilsofwar]

I never heard of a deep cycle AGM batter that would allow 80% repeated discharges.
This is for a very small watercraft. I don't expect a small unit to charge them from a significant discharge.

Can/should I just leave the small solar panel on and connected directly all the time, even when running electronics?
or maybe I can just let it discharge to 60% by day 3 and let it sit for a few days? letting an AGM sit discharged at even 10%-40% for a few days is bad, right?

Because you've never heard of it, it isn't a thing?

"Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are SOLID Lead plates - not sponge. This gives less surface area, thus less "instant" power like starting batteries need. Although these can be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 50% discharge."

Source: http://www.solar-electric.com/deep-c...ttery-faq.html

I could quote others.

Second, what do you not understand about what you're proposing? That tiny solar panel will not CHARGE your battery in any meaningful way. It will MAINTAIN it, against self-discharge, assuming everything else in the world is right (no parasitic load, sunlight sufficient, etc).

You say "very small watercraft"... How should we know what that is? What is the battery make/model? What is the main propulsion?

 

[spoilsofwar]

"A deep cycle battery on the other hand has much thicker plates and they are solid, not sponge. These thicker plates have less surface area and thus less of the instant power that a starting battery needs. They are designed to be discharged down to 20% of their maximum charge repetitively. The thicker lead plates allow for this as they are much sturdier than their sponge counterparts"

www.batteriesinaflash.com/deep-cycle-battery-faq

"Car batteries are especially designed for high initial cranking amps (usually 200 to 400 amps for five to 15 seconds) to start a car and for shallow (10% or less) discharges. They are not designed for deep cycle discharges. Deep cycle (and marine) batteries are designed for prolonged discharges at lower current and not for high current discharges. The plates in a car battery are more porous and thinner than in deep cycle batteries and use sponges or expanded metal grids instead of solid lead. A deep cycle battery will typically outlast two to ten car batteries when used in deep cycle applications. In warm weather, starting an engine will typically consume less that 5% of a car battery's capacity. In contrast, deep cycle (or marine) batteries are used for applications that will consume between 20 and 80% of the battery's capacity."

Www.marine-electronics.net/techarticle/battery_faq/b_faq.htm

 

[bruceb58]

With a true Deep Discharge battery like a Trojan flooded lead acid, you should still not discharge them past 50%. You can if you want but the capacity of the battery degrades faster.

I have 2 Odyssey AGMs on my Wellcraft. I never discharge them past 50%. I just want them to last longer.

 

[ondarvr]

You can leave that small panel connected all the time, the electronics won't even know it's there, and in reality the batteries won't either.

 

[spoilsofwar]

It has already been stated that the more gentle you are on a battery, to include a deep cycle battery, the longer it will last as a viable power source. The same applies to almost anything in regards to wear vs. output. This does not mean that deep discharge batteries are incapable of performing their torturous task of delivering power for long periods of time and over long timespans deep into their reserves without charging.

http://Www.interstatebatteries.ca/wp..._Printable.pdf

Www.batteryuniversity.com/learn/article/absorbent_glass_mat_agm

 

[bruceb58]

Yep....if you discharge to 75% vs 50%, you cut the useful number of battery cycles from 1000 to 550 according to the Interstate chart.

 

[bruceb58]

I just sent Odyssey a question asking them what their recommended max depth of discharge is.

 

[bruceb58]

The reality of the 7.5W panel is that it is rated at its open circuit voltage which is usually 17V. Therefore, you are looking at around 400mA. Definitely worth having it attached to the battery while you are discharging it. What brand and model are your AGM batteries?

 

[spoilsofwar]

Whatever odyssey says, in reply, you won't find a finer battery, I've been flogging a sears platinum (aka, their rebranded odyssey AGM) for 4 years now, down to God-knows-what percent discharge, and it never lets me down. Unfortunately, they don't sell them anymore.

 

[bruceb58]

Whatever odyssey says, in reply, you won't find a finer battery, I've been flogging a sears platinum (aka, their rebranded odyssey AGM) for 4 years now, down to God-knows-what percent discharge, and it never lets me down. Unfortunately, they don't sell them anymore.

Seriously? That sucks. That is what I use!

I see they carry an AGM called the DieHard Gold. I wonder who makes that one.

 

[spoilsofwar]

Seriously? That sucks. That is what I use!

I see they carry an AGM called the DieHard Gold. I wonder who makes that one.

Yep, I went to buy another this year, and found they were out of production. Easy enough to just buy an odyssey, but they are almost twice the price. After some research, I think these are the closest to (i.e., I think they are made by) the Odyssey's :
https://www.batteriesplus.com/batter...4m/sli34agmdpm

 

[bruceb58]

Yep, I went to buy another this year, and found they were out of production. Easy enough to just buy an odyssey, but they are almost twice the price. After some research, I think these are the closest to (i.e., I think they are made by) the Odyssey's :
https://www.batteriesplus.com/batter...4m/sli34agmdpm

yep..they look like the same case.

 

[nicknight]

Yep....if you discharge to 75% vs 50%, you cut the useful number of battery cycles from 1000 to 550 according to the Interstate chart.

Good chart.
thanks guys. I think I am going to leave the electronics off, I don't need them that bad.
I will leave the solar panel on whenever I do use them on other day trips though. The ~400mA@12V (in sun) does go a long way to supplying the 750mA max electronic load. A larger panel with a regulator and charge controller will be better when I get around to it.

 

[dennis461]

The solar panel is too small. Panel ratings are at high noon, bright sunny day, middle of summer, aimed directly at the sun.
Any deviations result in less output.
I use a 15 watt panel and may add a second, and that us for charging only, nothing turned on in the boat.
.

 

[bruceb58]

Odyssey replied back with this chart. Looks like discharging them to 80% is not a great thing to do. I am going to be rethinking even 50%. Fortunately, I don't discharge them even to 50% very often.

They also replied back with this:

Shallower cycling means more overall energy available over the life of the battery. Example: at 80% depth of discharge, the battery will give 400 cycles. At 50% DOD, about 620. And at 35% DOD, about 1000 cycles. For a 100Ah rated battery, that equates to 80Ah output for 400 cycles or about 32000Ah total over the life of the battery, 50Ah for 650 cycles or 32500Ah total or 35Ah for 1000 cycles or 35000Ah total. At less than 35% DOD it increases significantly.

It also assumes proper charge maintenance. There is more information provided in the technical manual concerning charge maintenance and there is a link to the compatible charger listing provided on the website Literature page as well. The minimum recommended charging current is 40% of the 10 hour amp hour rating of the battery or battery pack. This means that for a 100Ah rated battery, the minimum recommended charging current is 40A. Even though this product is an AGM product, that does not mean all AGM compatible chargers are compatible with ODYSSEY batteries.

Please contact me if you have any questions or concerns. Sincerely,