Do I really need a separate starter battery

[Zzmeyer]

Your input has been most helpful. Your configuration is exactly what I considered for my current boat. My alternator is a bit bigger than yours, but as you said, it couldn't handle the lithium charge.

[Davidla]

No matter the size of your Alternator just make sure it has an external regulator and you can use it. They just normally cost more than a DC-DC charger.

[goboatingnow]

Originally Posted by Davidla

No matter the size of your Alternator just make sure it has an external regulator and you can use it. They just normally cost more than a DC-DC charger.

Converting an existing alternator to an external regulator is not a simple task. There are now alternator dc dc converters that in effect do the same thing whilst leaving the stock regulator in place.

[Davidla]

Originally Posted by goboatingnow

Converting an existing alternator to an external regulator is not a simple task. There are now alternator dc dc converters that in effect do the same thing whilst leaving the stock regulator in place.

I didn't suggest converting an existing alternator and what you call a DC to DC converter I call a DC to DC charger, me thinks.

[Dieseldude]

I have no experience with charging lithiums, so can offer nothing about this. Using a lithium to quick charge a lead acid might work, but it is not desirable. Lead acid batteries are damaged by quick charging. It may not be noticeable on each particular instance, but if done repeatedly will cause early battery failure. The plates can warp due to excessive internal heat. Small projections can develop on the plates. The extra heat can evaporate electrolyte. Warping and projections will result in short circuits between plates when they touch each other. If quick charging is done only a few times to resolve an emergency, you may never develop shorted plates. But doing so repeatedly will eventually cause this failure.


Modern "smart chargers" are designed to manage charge rate and optimize charge time versus over heating and over charging. As noted in my previous post, an independent starting battery isolated from the house battery with an isolator is the most sensible and safest arrangement. One poster even went a step further with a second alternator. One to charge the starter battery, and one to charge the house battery. This eliminates the isolator which can also fail. Investing in an independent starting system is well worth the cost to get propulsion in an emergency and avoid the inconvenience of just not being able to move when you want to.


I am assuming that forum members understand the difference between deep cycle and starting batteries. The prospect to interchange duty is limited. A starting battery will not run house load very long. Deep cycle batteries may not dump an intense amount of current required to turn a larger diesel starter unless you have unusually large capacity. For starting batteries, this is the cranking amp rating. But this varies with temperature and drops off with age. Starting with deep cycles may work for a small engine though. If your engine has decompression valves to ease starting, opening them to start with the house battery might help. Aboard small vessels it may be challenging to find room for a large enough house battery bank to crank a large diesel.

[goboatingnow]

Originally Posted by Davidla

I didn't suggest converting an existing alternator and what you call a DC to DC converter I call a DC to DC charger, me thinks.

No you used the term external regulator which means a very specific thing in relation to an alternator.

[goboatingnow]

Originally Posted by Dieseldude

I have no experience with charging lithiums, so can offer nothing about this. Using a lithium to quick charge a lead acid might work, but it is not desirable. Lead acid batteries are damaged by quick charging. It may not be noticeable on each particular instance, but if done repeatedly will cause early battery failure. The plates can warp due to excessive internal heat. Small projections can develop on the plates. The extra heat can evaporate electrolyte. Warping and projections will result in short circuits between plates when they touch each other. If quick charging is done only a few times to resolve an emergency, you may never develop shorted plates. But doing so repeatedly will eventually cause this failure.

Lead acid batteries unlike lithium’s have a moderating charge acceptance value , so charging from any power source , once the voltage is appropriate doesn’t make any difference. Just like a 100A battery charger and a 20 A battery charger can both charge a small lead acid.

Allowing the wrong voltage and hence current flow can of course cause heating in any battery technology

Because lithium’s have a huge acceptance charge connecting lithium’s of differing states of charge is NOT a good idea. The same is not true of lead acid technology

Modern "smart chargers" are designed to manage charge rate and optimize charge time versus over heating and over charging. As noted in my previous post, an independent starting battery isolated from the house battery with an isolator is the most sensible and safest arrangement. One poster even went a step further with a second alternator. One to charge the starter battery, and one to charge the house battery. This eliminates the isolator which can also fail. Investing in an independent starting system is well worth the cost to get propulsion in an emergency and avoid the inconvenience of just not being able to move when you want to.

Smart chargers implement the bulk absorption cycle , but they do not typically limit anything they will if the battery accepts it charge at up to their full capability. Loads determine things not sources.

I am assuming that forum members understand the difference between deep cycle and starting batteries. The prospect to interchange duty is limited. A starting battery will not run house load very long. Deep cycle batteries may not dump an intense amount of current required to turn a larger diesel starter unless you have unusually large capacity. For starting batteries, this is the cranking amp rating. But this varies with temperature and drops off with age. Starting with deep cycles may work for a small engine though. If your engine has decompression valves to ease starting, opening them to start with the house battery might help. Aboard small vessels it may be challenging to find room for a large enough house battery bank to crank a large diesel.

The practical reality is unless you invest in 2V circular plates traction batteries what you get in “ deep cycle” batteries with “ marine “ slapped across them is in effect cranking batteries. Some will have ticker plates but the reality is they are all cranking batteries at heart

( my 300 aH domestics are big truck cranking batteries. I get 5-7 years )

Hence the practical reality is almost all lead acid on board a boat( even tubular traction ) will happily crank a small -medium diesel.

Lithium’s ( in the main ) make excellent cranking batteries as the plethora of emergency start systems will testify to. The issue can come if their are restrictions due to BMS specifications.

We all agree that an alternative method of cranking your diesel should be available , I can jump my starter to my domestics if needed and I carry a USB rechargeable Lithium jump start as backup to the backup.

Dual alternators is a waste of time especially dedicating one to a battery , the starter , that rarely needs much charging , car alternators are not there to charge batteries that’s an aside they are present to provide power to the cars systems. Dual alternators might be useful to balance power take off loads or provide extra domestic charging if space is restrictive

Hence there are several ways modern technology can be deployed to provide a backup starting solution , all of them have benefits and drawbacks , most of which come down to personal preference. There’s is no one “ right “ way.

[Bycrick]

Look at the number of threads that advocate that the navigation solution is a basket full of GPSs. Or multiple alternators. Or multiple [fill in the blank]. Why is there any question that you might want an emergency/reserve/starter battery when unexpected things happen?

[goboatingnow]

Originally Posted by Bycrick

Look at the number of threads that advocate that the navigation solution is a basket full of GPSs. Or multiple alternators. Or multiple [fill in the blank]. Why is there any question that you might want an emergency/reserve/starter battery when unexpected things happen?

Yes but most of the alternative gps these days are there by default. Most boats will have one additional GPS as part of the safety strategy

We all agree there should be an alternative engine start ability.

That doesn’t have to be the traditional one however.

[Don C L]

Originally Posted by goboatingnow

I have two battery monitors. How does anyone “ wake up “ to a dead battery these days. Answer cause they are incompent or couldn’t be bothered to install a monitor.

In the last 20 , yes twenty years , I’ve never had an unexpected battery failure or unexpected dead battery situation. I can see batteries slowly failing etc. But not the “ wake up “ to a dead battery.

That’s not a failure sceanario that’s just incompetence quite frankly

These days a separate lithium jump start option is widely available so systems that don’t want a permanent starter battery can easily carry a backup. I do agree an alternative method should always be available but that does not excuse ending up with dead batteries

Clearly I need better friends!

[goboatingnow]

Originally Posted by Don C L

Clearly I need better friends!

Just ones with battery monitors

[Dieseldude]

Thank you Goboatingnow for your additional insight into battery and charging systems. But starting batteries certainly need charging after each start, or they will be depleted. And that is an important purpose of an alternator. I'm sure that all forum readers can relate to dead vehicle and boat batteries after multiple start attempts of an engine that just won't start due to some fault. Dual alternators are an added expense, but a good isolator in a single alternator system is also an expense. Well designed smart chargers will limit charge current to protect batteries from effects of excessive charge current. My portable that runs off 120 volts AC has settings for about 3, 6. and 12 amps. Class approved chargers for large vessels have quite complex monitoring and control.



The degree of redundancy in any system is always a balance of cost, complexity, safety, available space, and convenience. A basic single alternator with dual battery and isolator should be fine for most boaters. And of course frequent checks and maintenance of the system is critical.


A forgotten detail of battery and charging systems is that a battery in the system will serve to smooth out spikes and ripples in a system. Battery plates act as capacitor plates to absorb transient voltages. If you ever try running on the alternator alone, due to a failed battery, after starting on a boost, you might possibly get trouble with sensitive electronics. I'm not saying that phantom electronic problems will happen in every case, but the possibility exists. The battery also prevents voltage and current sags when the alternator takes brief high demands. A battery should always be in the circuit for the sake of sensitive electronics and transient loads.

[goboatingnow]

Nobody is arguing there isn’t one battery in the system.

Nobody is arguing there shouldn’t be an alternative energy source to start your engine.

The issue after that is largely personal preference ( or more likely predilection ) , solutions can also be space or budget constrained.

For example I don’t have a dual battery switch. I have a jump lead. My solution provides exactly the same facility without the need for a switch of that nature ( many of whom have failed unexpectedly ). I have no problem in jump starting my diesel from the domestics. ( cause my starting bat died and I started the engine a few times with the jump leads )

My alternator does not charge my starter battery my dc dc charger from my domestic does that.

Equality I have a lithium Jump start. To me that much more useful then a secondary alternator

I would argue my solution provides as great if not more redundancy then many others just arranged in a different way.

[goboatingnow]

Note I have a sophisticated 4 phase battery charger with temp monitoring. The charger adjust the charging voltage to limit the battery currents in certain circumstances. But the fact remains that lead acid charge acceptance acts as a automatic control system ( ie rising equivalent Thevevin resistance as the charge cycle continues)
Hence a charger set at the right voltage can simply feed as much as the battery desires without issue in lead acid. This is not the case with lithium’s.

This is why car alternators at set at around 13.8 v irrespective of the battery or alternator size. Of course increasing the voltage can result in overwhelming current causing gassing and heating but that’s a different thing.

[Dieseldude]

Goboatingnow has a basic system that he is happy with, and that is effective. That's the bottom line. The four phase charger is a useful enhancement. I have had to rig jumper cables and that is fine in a leisurely situation. But rigging jumper cables in an emergency can cause loss of valuable time. Clamps can pop off terminals and poor contact can cause problems. High quality cables with flexible insulation and rugged clamps can avert such problems. I prefer a hard wire system with a switch. Of course the cables can serve as a back up for this.


A charger equipped with heat sensing at the battery might seem like a fancy addition, but it will protect from over charge damage. If excessive heat is detected, the charger shuts down or limits current. Even some modern vehicles have a temperature sensor in the battery shelf. I have seen costly destruction of batteries in an old basic system that failed. Batteries over charged and "boiled dry". In addition to the direct loss, this was a fire and explosion hazard, and could have been a health hazard with chance of acid vapor. Well designed modern charge systems over time can pay off in preventing such costly failures and hazards.


I would be interested in more info the Thevevin resistance automatic control. Any good web link on this ? It is common theory that battery cell internal resistance increases as a battery discharges and decreases as charge increases. This would tend to cause charge current to increase toward completion of the charge cycle, quite the opposite of what one would want. Of course voltage equalization will eventually decrease charge current as battery voltage reaches charger voltage. This causes less current demand of the battery that is being charged. If battery voltage reaches charger voltage, in theory, no current is exchanged. The electrons lose incentive to move when voltages are equal.